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: improve colours

* make the background lighter
* set highlight colour to something readable
* improve readability of default grey colours
* make default text color readable
* fix some base colours
style option control in light theme
fix gap between input and content
Fix bug with background color bleeding through
Merge pull request #434 from glowing-bear/design-light-theme

Design light theme