luci-theme-bootstrap: light/dark mode releated CSS fixes

 - Fix progress bar shadow in light mode
 - Fix color schema preference in light and dark modes

Fixes: #5498
Fixes: #5499
Fixes: ed86f03a

 ("luci-theme-bootstrap: add explicit dark/light mode selection")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>