This allows for users to change the colors if needed (may be useful
with some themes or color blind persons).
On the sidebar, only the color is applied for now. This is because
it is not possible to style cell renderers through RC files, all having
to be done in the code; so currently only the color is applied.
This allows the user to override the custom styles we apply to some
widgets, like e.g. the unmatched search entry colors.
We use the :theme priority rather than the :application one because
it seems that the :application one cannot override theme settings, even
if it matches against a name the theme don't have rules for but have
rules for the class of that widget. This prevents a theme from
overriding our styles, but it's unlikely a theme actually provide some
specific stuff for us anyway.
