Improve poll meter bar contrast in dark mode

[jensfr1]

What

In dark mode the poll result bar (.pollMeterValue) was filled with a
faint translucent white (rgb(255 255 255 / 34%)), which is hard to
distinguish from the underlying track. This makes it difficult to read
the relative share of votes at a glance.

This changes the dark mode fill to the accent color (--wcfContentLink),
so the filled portion is clearly visible and consistent with the
appearance of the bar in light mode.

Why

Reported in #5925 with before/after screenshots.

Notes

Single-line style change in wcfsetup/install/files/style/ui/poll.scss.
Using the --wcfContentLink variable keeps the color in sync with the
active style's accent instead of hardcoding a value.

[dtdesign]

This is actually a worse contrast ratio plus you are using a variable that you have no control over in terms of it matching the underlying bar. The problem with contrast is, that the perception is misleading because it needs to account for different visual impairments.

It’s easy to think of it as a binary “blind – not blind” where in reality this is not only a gradient but also a mix-and-match of different impairments, for example the inability to perceive color. To better illustrate the problem, here is a screenshot of the referenced issue but converted to grayscale: