Specify `font_size` as s, m, or l in `params.toml`.
Font size is now a site setting rather than theme setting.
Also:
Be less strict on user input of theme and font set names - allow lower
or upper case as well as spaces.
Fall back to Minimal theme and font set on user input error.
Keep category icon on same line as category links, otherwise context
lost.
Change search-header position in modal now that it overlays entire page
*including* navbar.
Clean up code, add some defaults, and add error messages when the
specified font or color theme cannot be found.
Refactor `text-muted` color from CSS override to Bootstrap variable.
Close#680
George Cushen