Make gruvbox i3xrocks values a brighter color

[natejlong]

I don't know what the protocol is for proposing changes to themes, but this is a change that I've made locally and liked. I thought the previous text color was too close to the background color to be easily readable at a glance.

Before:

After:

[kgilmer]

Certainly an improvement, thank you for taking the time to put together a PR @natejlong

[natejlong]

Thanks for merging!