Fix contrast in new dark mode code block

[HarHarLinks]

Is your suggestion related to a problem? Please describe.

Congrats on the snazzy looking new code block styling. In the above screenshots it looks pretty good!

Now look at this message:

My eyes try to skip the code snippet as it is not only far from the text color but also darker than normal text (less fg-bg contrast) while the background is lighter than normal background (next to no contrast left).

Describe the solution you'd like.

Notice how in light mode, the non-coloured text has the same color as regular messages. In dark mode, the same parts in code are kind of faded. I'd like if they matched the normal message text color, or much closer to it, especially inline.

Describe alternatives you've considered.

🤔

Additional context

Screenshots taken on nightly 2021071001-1

[HarHarLinks]

I think this might as well be a T-Defect at this point

Edit: for reference, also in 2021071101-1

[HarHarLinks]

If this still happens, I don't consider this resolved

[SimonBrandner]

If this still happens, I don't consider this resolved

On it: matrix-org/matrix-react-sdk#6355

[HarHarLinks]

this is better as of today's nightly, but now there is a horizontal scrollbar

something something bubbles regression?