Fix regression after 117197e11d
where space or enter would not trigger a click on
link outside of the main window.
".keyboard-clickable" is handled by listener in ZoteroPane,
so links outside of it (e.g. preferences) are not affected by it.
For now, just explicitly handle "keypress" event by the
zotero-text-link component.
Also, added a focus-ring around the link so it is visible
when it is focused.
Fixes: #4521
* Opt-in for light and dark color schemes
* Convert scss variables in _light.scss to css variables
* Add _dark.scss for dark color scheme
* Remove unused, theme-related variables
