fix(ui): fix settings save error and dark theme select readability

Allow empty string values in Settings::save() — settings like
set-regex-mode and set-text-visit-statuses-via-key use empty string
as a valid option ("Default"/"Filter off"), causing 500 errors on save.

Add dark background/foreground styling to <option> elements across all
dark themes (Dark, Dark_Muted, Underline_Dark) and base dark mode.
Replace hard-coded light-mode colors in searchable-select's .is-selected
state with CSS custom properties that adapt to the active color scheme.

Author: HugoFara