This commit abstracts the theme-related code from komorebi-bar into a
separate library which is now also consumed by komorebi.
The static configuration file for komorebi has been updated to allow
users to specify themes and provide palette overrides for various border
styles and stackbar configuration options.
In the event that both a theme and border-specific and/or
stackbar-specific colours have been specified, the theme will take
priority to make it as easy as possible for users who have already spent
time tweaking their colours to try out themes and quickly revert back if
they prefer their existing colours.
This change makes it easier for users to have unified themes between
komorebi and the komorebi status bar.
