Resolve "Update colors on billing page statistics on dark mode"

What does this MR do and why?

Improves the contrast of the text on the billing page when the dark theme is enabled.

There were a few miss configured SCSS properties and a missing definition of a colour used for the active cards background.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #376426 (closed)