[Theme] Arc Green needs a little more love

Opening an issue here since theme repo now deprecated with the inclusion of arc-green into the main codebase.

New theme looks great but has some usability issues which need to be addressed:

Wiki formatting toolbar icons invisible

Original issue: https://github.com/go-gitea/gitea-darkmode/issues/6
Unmerged PR: go-gitea/gitea-darkmode#8

Zebra striping in README tables unreadable

Original issue: https://github.com/go-gitea/gitea-darkmode/issues/3

Highlight.js Improperly rendering fenced code blocks

Original issue: https://github.com/go-gitea/gitea-darkmode/issues/4

@techknowlogick the updates look fantastic and I see you also darkened the background on the wiki edit page which was a bit of an eyesore. in review I see the zebra striping hasn't carried over to tables on the wiki pages themselves (example). want me to open another issue for that?

[techknowlogick]

@JHabdas a new ticket is good, but a new PR would be even better.

Sounds good. I'll send a pull up after I find a few more nits.