fix(scss): respect dark mode in table-head-fixed (#6026)

The sticky table header was using background-color: \$white (a fixed
SCSS variable), so dark mode left it as white-text-on-white-background.

Swaps the fixed colour for Bootstrap's CSS variables so the sticky
header tracks the active colour scheme automatically:

- background-color: var(--bs-body-bg, #{\$white})
- box-shadow inset: var(--bs-border-color, ...)

The fallback to the SCSS defaults preserves behaviour if AdminLTE is
consumed without Bootstrap's CSS variables in scope.