UIkit .uk-select option text is set to #666 by its base rule while the
.app makes the control dark through .uk-background-secondary / uk-light
and declares `color-scheme: dark` on :root. Firefox paints the native
listbox surface dark; the #666 option text then has almost no contrast,
so Category / Android version filter values are invisible (Chromium
paints its own surface and was not affected).
Give the footer's single selects explicit option foreground/background
matching the app's dark theme, with a selector at least as specific as
UIkit's `.uk-select:not([multiple]):not([size]) option`.
Refs: playsearch.kaki87.net/v2-web#40
