kcm/colors: don't dull accent colour on dark themes in colorsapplicator

User demand shows that bright accent colours are wanted over potentially
better legibility, so let's avoid changing them.

BUG: 442820
FIXED-IN: 5.25
(cherry picked from commit 6750353c)
2 jobs for Plasma/5.24 in 8 minutes (queued for 5 seconds)