I just made light and light blue themes for Darktable 3.0. I recommend “Poppins” font.
Theme Files: GitHub - abdullahoguk/darktable-light-theme
1. Place themes and icons folders into ~/.config/darktable directory.
2. Open Terminal and run these commands to create a link to main themes directory of Darktable.
cd ~/.config/darktable/themes
ln -s /usr/share/darktable/themes/
3. (Optional) Place fonts in in poppins-font folder into ~/.fonts directory.
Looks very cool, I like the readability of the fonts and the contrast. Modern and clear.
But you should know that a colored UI deceives your perception of colors in the image while you work. That reduces your ability to assess colors.
Can you make a version in grayscale?
@pphoto At first I started to make just a light theme. I decided to add some color to make selected images more distinguishable, and the rest came.
I desaturated values of blue color variables. You can duplicate darktable-elegant-light and replace the definititions with these.
@define-color blue_black #404040;
@define-color blue_blacker #383838;
@define-color blue_dark #383838;
@define-color blue_darker #383838;
@define-color blue_light #828282;
@define-color blue_lighter #919191;
@define-color blue_white #dedede;
@define-color blue_whiter #f7f7f7;
@define-color blue_middle #808080;
@define-color blue_middler #737373;
Hello @abdullahoguk
Thanks a lot for your themes!
Just tried your new suggestions (Windows 10 - standard font as previuosly done) 
You shouldn’t need to duplicate the whole file. Instead just import it using CSS’ import functionality. You can call this file poppins-darktable.css:
@import "darktable-elegant-light";
@define-color blue_black #404040;
@define-color blue_blacker #383838;
@define-color blue_dark #383838;
@define-color blue_darker #383838;
@define-color blue_light #828282;
@define-color blue_lighter #919191;
@define-color blue_white #dedede;
@define-color blue_whiter #f7f7f7;
@define-color blue_middle #808080;
@define-color blue_middler #737373;
Thank you @paperdigits .
@pphoto I fixed the issue and updated the files.
Thanks very much for the themes!
Just a small note.
The files need to be placed in in program folder to work. If you them in the user config folder in a “themes” subfolder, they are visible from the gui, but do not work.
I guess it’s because the “import” statament requires the included files to be place in the same folder.
I guess the user shoud normally not touch the program folder, or ?
@MarcoNex 
That should be the right way. I solved import issues by creating a symlink of program’s theme folder in user config folder and modified the imports.
Nice, but if you use a blue theme for photo editing. It is likely that all your photos you process will have a color cast as you will try to compensate the blue!
Just be warned 
I agree – there’s a reason most raw converters and photo editors keep the UI colour scheme mostly monochromatic. Personally I like the default “darktable” theme, and I set a larger border size for the 50% grey margins around the image preview. The all 50% grey is too light for me because I mostly edit in a dark room, so it’s too intense for me.
If you only output to white border prints or white online gallieries (e.g. Instagram), changing the borders to white (in General Preferences) might be a good idea as well.