Topics Link and Button Contrast in the Dark Theme

Contrast is mentioned in #1530 (New UI is unreadable) but that's for multiple themes and primarily about the light.

It may help to note contrast and colour issues that are dark-specific. I suggest adding this to the Dark Theme Updates milestone and applying the accessibility label.

Gitter Topics beta

Dark grey text on a button that has a dark grey background. Insufficient contrast:

Dark blue link text on a dark grey background. An unpleasant combination, with insufficient contrast:

Dark purple link text on a dark grey background. Insufficient contrast:

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There are other issues, within and beyond Topics; the three above were the most obvious within the first few minutes of browsing Topics.