theme: Split shared style into shared and fallback

The fact the shared style is loaded at override priority is causing
problems for elementary: Librem5/libhandy#273

While window corners are fixed by the previous commit, they aren't the
only area that they may want to style. Split the part that's just visuals
into a separate stylesheet named fallback.css, and load it at a fallback
priority for all stylesheets, leave shared for critical styles, such as
12 jobs for wip/exalm/fallback in 16 minutes and 17 seconds (queued for 1 second)