Create dark theme by extracting theme settings from glowingbear.css

* Remove style.css as those values should be themable as well.
* Update urls in index.html to link to dark theme by default
Add light theme
light-theme: darken most colors by 50% and add it to theme manager
[light theme] make the background lighter
[light theme] set highlight colour to something readable
[light theme] improve readability of default grey colours
fix: fix the notification colour again
[light theme] make default text color readable
themes: fix some base colours
style option control in light theme
fix gap between input and content
Fix bug with background color bleeding through