I made a flat-style one for myself, but worth to check https://github.com/Berikai/awesome-bitwig-themes for more

Code:

{
  "window": {
    "Abstract Button Pressed Background": "#2a2a2a",
    "Abstract Button Selected Background": "#2a2a2a",
    "Abstract Button Stroke": "#3a3a3a",
    "Abstract Button Unselected Background": "#101010",
    "Activation Green": "#26d96f",
    "Activation Red": "#7d3a3a",
    "Activation Yellow": "#e5d538",
    "Active Inverted Toggle Icon (Playing)": "#e0e0e0",
    "Active Panel stroke": "#2e2e2e",
    "Active Toggle Icon (Playing)": "#e0e0e0",
    "Analyzed Beat Color": "#478a8a",
    "Audio Event Boundary": "#191919",
    "Automation Color": "#597893",
    "Beat Marker Color": "#7d3a3a",
    "Bitwig 16-Track": "#0d4a7d",
    "Bitwig 8-Track": "#33e4c2",
    "Bitwig CI": "#ff6010",
    "Bitwig Essentials": "#2a4a7d",
    "Bitwig Mint": "#3094d9",
    "Bitwig Producer": "#ffcf43",
    "Bitwig Red": "#7d3a3a",
    "Button background": "#3a3a3a80",
    "Button in tree background": "#2a2a2a",
    "Button stroke": "#2a2a2a80",
    "Checkbox background": "#424242",
    "Close button mouse over background": "#303030",
    "Close button pressed background": "#404040",
    "Comp fill": "#007f7f",
    "Content Background": "#282828",
    "Dark Panel Sub-frame Fill": "#3a3a3a",
    "Dark Panel Sub-frame Stroke": "#191919",
    "Dark Separator Line": "#424242",
    "Dark Text": "#0e0e0e",
    "Dark tree background (selected)": "#2a4a7d",
    "Dark tree background (standby selected)": "#224343",
    "Dark tree hover background change": "#e0e0e035",
    "Dark tree separator": "#424242",
    "Dark tree text": "#ececec",
    "Dark tree text (selected)": "#ececec",
    "Default text": "#000000",
    "Device Header": "#303030",
    "Device Header (selected)": "#343434",
    "Device Locked Overlay": "#0e5ca180",
    "Device Tint Future": "#58859564",
    "Device Tint Military": "#305d0247",
    "Device Tint Retro": "#c07b4c52",
    "Display Background": "#191919",
    "Display Background (error)": "#482a2a",
    "Display Loop Markers": "#00b4a0",
    "Display Start/End Markers": "#7d7d0f",
    "Display Waveform": "#ececec",
    "Drop Indicator": "#ececec",
    "Emboss Highlight": "#ececec",
    "Emboss Shadow": "#0000003c",
    "Error Text": "#7d2525",
    "Field background": "#a6a6a6",
    "Grey Display Background": "#222222",
    "Grid Line (Primary)": "#ececec",
    "Grid Line (Secondary)": "#e6e6e6",
    "Hitcount background": "#5c5c5c",
    "Hitcount text color": "#e0e0e0",
    "Hitech background": "#223434",
    "Hitech on": "#3ebaff",
    "Hole (dark)": "#171717",
    "Hole (light)": "#323232",
    "Hole (medium)": "#2a2a2a",
    "Implicit On (subtle)": "#ee821d",
    "Inspector Section Frame": "#222222",
    "Inverted Selected Borderless Button background": "#2a2a2a",
    "Knob Body Darkest": "#535353",
    "Knob Body Lighter": "#5c5c5c",
    "Knob Body Lightest": "#767676",
    "Knob Emboss Highlight": "#a6a6a400",
    "Knob Emboss Shadow": "#191919",
    "Knob Line": "#e0e0e0",
    "Knob Line Dark": "#191919",
    "Knob Stroke": "#0c0c0c",
    "Knob Value Color": "#7d5010",
    "Launcher Mapping Indication": "#424242",
    "Led Off": "#5c5c5c",
    "Led On": "#7d1010",
    "Light Separator Line": "#5c5c5c",
    "Light Text": "#e0e0e0",
    "Light Tooltip Background": "#a6a6a6",
    "Lighter Text": "#ececec",
    "Link Text": "#7d1010",
    "List Item Background": "#a6a6a6",
    "List Separator": "#5c5c5c",
    "Mapping": "#007d5f",
    "Mapping indication 1": "#7d3a3a",
    "Mapping indication 2": "#7d502a",
    "Mapping indication 3": "#7d7d10",
    "Mapping indication 4": "#356a1a",
    "Mapping indication 5": "#356a5a",
    "Mapping indication 6": "#3e5e8a",
    "Mapping indication 7": "#563e7d",
    "Mapping indication 8": "#7d5a7a",
    "Medium Light Text": "#b0b0b04",
    "Menu Icon": "#424242",
    "Menu background": "#222222",
    "Menu description text": "#5c5c5c",
    "Menu separator": "#323232",
    "Menu stroke": "#0c0c0c",
    "Menu text": "#e0e0e0",
    "Meter Clipping": "#ff0000",
    "Meter Gain Reduction": "#5386b6",
    "Meter Hitech": "#23d1d4",
    "Meter Hitech Background": "#23343e",
    "Meter Muted": "#979797",
    "Meter Normal": "#8ccb2a",
    "Meter Red": "#ff8b00",
    "Meter Yellow": "#ffde42",
    "Modern Dark Scrollbar Handle (active)": "#191919",
    "Modern Dark Scrollbar Handle (inactive)": "#191919",
    "Modern Scrollbar Background (active)": "#262626",
    "Modern Scrollbar Handle (active)": "#424242",
    "Modern Scrollbar Handle (inactive)": "#424242",
    "Modulation Mapping Background (polyphonic)": "#34e08c2d",
    "Modulation Mapping Background Color": "#3dd9ff32",
    "Modulation Mapping Color": "#39d0ff",
    "Modulation Mapping Color (polyphonic)": "#34e08c",
    "Modulation Mapping Color (subtractive)": "#567e91",
    "Monitoring buttons color": "#e1d510",
    "Mouse Over Inverted Toggle Icon": "#191919",
    "Mouse Over Toggle Icon": "#424242",
    "Normal Inverted Toggle Icon": "#00000060",
    "Normal Toggle Icon": "#828282",
    "Notification Background": "#a6a6a6",
    "Notification Error": "#7d1a1a",
    "Notification Normal": "#006d6d",
    "Number field bar background": "#7d5010",
    "On": "#e08420",
    "On (subtle)": "#ea7e1d",
    "On (subtler)": "#ec7f1d",
    "Onset Marker Color": "#2a4a7d",
    "Panel Blue": "#3e5e8a",
    "Panel Green": "#41b600",
    "Panel Lime": "#b4e21e",
    "Panel Mint": "#26d098",
    "Panel Orange": "#e68127",
    "Panel Purple": "#704f8f",
    "Panel Red": "#7d3a5a",
    "Panel Turquoise": "#287f7f",
    "Panel Yellow": "#ffda42",
    "Panel body": "#303030",
    "Panel stroke": "#222222",
    "Popup Notification Background": "#191919",
    "Popup insert": "#525252",
    "Popup overlay background color": "#222222",
    "Pressed Inverted Toggle Icon": "#191919",
    "Pressed On": "#d46d20",
    "Pressed Toggle Icon": "#666666",
    "Pressed button background": "#404040",
    "Pressed view button background": "#404040",
    "Progress background": "#244a6e",
    "Progress bar": "#2a4a7d",
    "Record button color": "#ec5c4c",
    "Rubber button stroke": "#2e2e2e",
    "Rubber highlight button stroke": "#101010",
    "Scrollbar": "#424242",
    "Scrollbar background": "#222222",
    "Selected Dashboard Tree": "#2e2e2e",
    "Selected Panel body": "#5c5c5c",
    "Selected Panel stroke": "#7a7a7a",
    "Selected Panel stroke (standby)": "#5c5c5c",
    "Selection": "#ececec",
    "Shadow": "#1b1b1b20",
    "Standby selection": "#d6d6d6",
    "Subtle Dark Text": "#5c5c5c",
    "Subtle Light Text": "#ecececa8",
    "Subtler Light Text": "#e0e0e03c",
    "The Grid (background)": "#101010",
    "The Grid (stroke)": "#191919",
    "Timeline edit tool chooser background": "#222222",
    "Tooltip Background": "#323232",
    "Tooltip Stroke": "#424242",
    "Tree Item Background": "#a6a6a6",
    "Tree Separator": "#191919",
    "User Automation Override Color": "#27d927",
    "View button background": "#404040",
    "Warning": "#7d2525",
    "Window background": "#222222"
  },
  "arranger": {
    "Add Modulation Button Color": "#bb33b1",
    "Audio connection in modular environment": "#c14524",
    "Audio64 connection in modular environment": "#d281f4",
    "Automation button glow color": "#93ff9240",
    "Background color of the modular environment.": "#000000",
    "Clip Automation Button Color": "#d93232",
    "Clip Automation Color": "#8d0a0a",
    "Clip Modulation Color": "#881390",
    "Compressed Audio Port in modular environment": "#ff8000",
    "Current Atom Value Color": "#005270",
    "Dark Timeline Background": "#2b2b2b",
    "Dark Timeline Header Background": "#323232",
    "Event connection in modular environment": "#3c7399",
    "Header Loop Region Background": "#00000032",
    "Header Time Selection Across All Lanes Fill": "#7e9898c8",
    "Header Time Selection Across All Lanes Stroke": "#dedede",
    "Header Time Selection Cursor Stroke": "#dedede",
    "Header Time Selection Fill": "#a7939dc8",
    "Header Time Selection Standby Cursor Stroke": "#b0b0b0",
    "Header Time Selection Standby Fill": "#ffffff06",
    "Header Time Selection Standby Stroke": "#a7a7a7",
    "Header Time Selection Stroke": "#f7f7f7",
    "Insert preview time": "#d2d2d2d2",
    "Irrelevant Timeline Background": "#333333",
    "Irrelevant Timeline Header Background": "#323232",
    "Irrelevant Timeline Header Overlay": "#ffffff0a",
    "Irrelevant Timeline Overlay": "#0000000a",
    "Light Timeline Background": "#3a3a3a",
    "Light Timeline Header Background": "#353535",
    "Loop Region Background": "#ffffff32",
    "Multiply Modulation Button Color": "#bb33b1",
    "Note Expression Color": "#055988",
    "Polyphonic Desktop Object": "#3c7399",
    "Record button color implicit": "#b02929",
    "Record button glow color": "#ff000040",
    "Selected matrix slot color": "#676767",
    "Send (post) value color": "#ffe019",
    "Send (pre) value color": "#4df4ff",
    "Time Selection Across All Lanes Fill": "#5e9898c8",
    "Time Selection Across All Lanes Stroke": "#f7f7f7",
    "Time Selection Cursor Stroke": "#ffffff",
    "Time Selection Fill": "#7dc3d05d",
    "Time Selection Inactive Fill": "#ffffff1e",
    "Time Selection Inactive Stroke": "#cecece",
    "Time Selection Not Selected Fill": "#00000000",
    "Time Selection Not Selected Stroke": "#cecece",
    "Time Selection Standby Cursor Stroke": "#f7f7f7",
    "Time Selection Standby Fill": "#ffffff46",
    "Time Selection Standby Stroke": "#cecece",
    "Time Selection Stroke": "#f7f7f7",
    "Timeline Background Pattern": "#00000014",
    "Timeline Cue Marker": "#535353",
    "Timeline Header Background Pattern": "#ffffff0a",
    "Timeline Header Cue Marker": "#545454",
    "Timeline Header Primary Grid": "#828282",
    "Timeline Header Secondary Grid": "#565656",
    "Timeline Header Tooltip Background": "#484848c8",
    "Timeline Header Tooltip Text": "#aaaaaa",
    "Timeline Primary Grid": "#00000064",
    "Timeline Secondary Grid": "#00000014",
    "Timeline Tooltip Background": "#998e8ec8",
    "Timeline Tooltip Text": "#2a2a2a",
    "Top Level Timeline Background": "#a6a6a6",
    "Top Level Timeline Header Background": "#323232",
    "Track Automation Button Color": "#4f9ddb",
    "Track Automation Color": "#055988"
  }
}