Adapt striped table override to dark mode

[AntonKhorev]

See #2332 (comment) for how striped tables currently look in dark mode.

This change keeps the current "striped" color in light mode, while in dark mode it looks like this:

Another option would be removing this override and using the standard colors.

[tomhughes]

Can you explain why this works? It's changing from a pale grey to a light purple but also using an alpha channel to make it mostly transparent?

Out of interest I looked at the default colours - that would give us this:

instead of this:

which I personally think is fine except possible that it makes the striping very close to the header colour - probably it's the secondary colour so would in fact be the same as the heading if we switch that to secondary in #4651.

[AntonKhorev]

One thing I didn't like about the default stripe color is that it's darker than bg-body-tertiary I tried to use for the heading in #4651.