start-here-kde: Make icon adaptable to dark themes

BUG: 416302
FIXED-IN: 5.91

Make the dark arrow part of the plasma logo change to white when used with a
dark theme. The two icon sizes are also made more alike color-wise.
The changes include:

- sizes 64 & 96 using the same kind of grey,
- the dark part being color adaptive to dark themes,
- size 64 no longer using the accent color for the bottom dot.