Active grey not visible on some monitors

@cassiocassio pointed to this dovetail user testing session, in which a user remarks that he doesn't get visual feedback when changing between segmented tabs - meaning, the active grey colour is not distinguishable from thee page background on his particular screen.

He makes a similar comment about code snippets. I think in the case of code snippets, it might be less of an accessibility concern, as they are not interactive, but our active states should definitely have a 3:1 contrast change between states, to ensure thery work monitors with a very limited range of contrast, as seems to be the case here:

[dovetail link](https://canonical.dovetailapp.com/data/77Cyr2KemiAvD1VxwVQDhH#:v:h=40LcnEiVlAp3xv1McCnuXg&s=1) (requires dovetail access)