Various dark mode problems

[nephele]

• Replicant dragging on the destop has badly visible lines, This likely needs a more comprehensive fix, since it intersects with the functionality of background images.

• Window top bars have gradients that are too bright

• BTab has gradients that are too bright

• BTab devider (between inactive tabs) is not visible

• pref/Appearence colors of font preview don't live update

• aboutSystem colors don't live update

• There is no visible border between TextFields and panels when both are the same color (also a light mode problem?)

• Button gradients are too jarring in dark mode, should be more gradual. (same with BTitleColumnView)

• Some Icons use Dark lines as design elements making those not visible (pref/appearence, pref/network, pref/sounds, demo/sudoku)

• Shine on window borders on the tab does not match the window (It looks like the window tab is one pixel too far on the left because of the shine)

Additonal problems reported by Nipos:

• Menu selected color is too bright (fix: ​https://review.haiku-os.org/c/haiku/+/7937)

• demo/Sudoko no support for dark mode

• pref/Keymap Buttons stay white

• pref/time Clock stays white

• pref/media Slider labels stay dark red

• demo/Cortex plugin list is dark on dark

• demo/font render area is always light