As of r38.2, you can now use custom themes in Construct 3! Our designer Paulo has been working on a dark theme for the Construct 3 editor. Here's a preview of what it looks like.
You can now download this theme and try it out for yourself! Click the link to download a .c3addon for the dark theme:
[ edit: now available as a built-in from r41 - just select it in 'Settings'! ]
Simply install this like any other addon in the Addon Manager, and restart C3. Then go to Settings and you should see the Dark theme listed under 'Themes'. Select the theme, close settings, and restart C3 again, and you should be using the new dark theme.
Note this is a beta of the dark theme - themes can be tricky to get exactly right and sometimes can introduce bugs or quirks. So please let us know if you find anything amiss and we'll work to make sure everything works well with the new theme!