frontend: kelp green is hard to see white text on

[ES-Alexander]

Current behaviour

Following on from #1606, @ArturoManzoli has mentioned that

I like the kelp green accent color (#4FA483), but I’m concerned about its contrast with the white text.
On my monitor, it’s a little hard to read, especially in very bright environments.

Expected or desired behaviour

Our main options are:

1. Redefine Kelp Green
   • I found this reference from a paint company, which has a "Kelp Forest Green" colour that's displayed as #00967A
2. Use a darker shade of the existing colour, just for interface elements with white text
   • This may look a little strange, because then the application icon would be a different colour
3. Stop trying to use Kelp Green in Cockpit, and switch to something else
   • We were previously using this random blue colour #3B78A8 as those accents, if that's relevant

I've raised these ideas with SMS, and will follow up if there's a decision made :-)

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.