Feature: add dark theme for web ui

[surfmuggle]

A dark theme is popular and also can be beneficial.

Quote from Dark Mode Can Improve Text Readability

Dark modes have numerous benefits for certain users:

 - If a screen has flicker problems, dark backgrounds can reduce flickering.
 - Dark themes are less likely to trigger photophobia (sensitivity to light).
 - Light-colored text on a dark background can be easier to read for a long period of time.
 - Dark themes can make screens easier to use in low-light conditions.

A quick hack using devtools - look ma it even uses lime