zellij: make ribbon_unselected background darker and base lighter

[muni-corn]

Fixes #143.

This PR modifies the colors of unselected ribbons so that they're more in harmony with the rest of the Zellij stylix theme.

Comparison (old on top, new on bottom):

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[0xda157]

this change causes the ribbon_unselected background to not contrast enough with the base when using catppuccin mocha for example (as tested in testbed:zellij:dark). additionally, when using the official zellij catppuccin mocha theme as an example, stylix's current implementation matches their implementation. ideally stylix would follow the upstream styling when possible.

[muni-corn]

heard! how does this look? i simply bumped base04 to base05 for base.

note: i don't believe this PR even addresses the linked issue as well. i didn't grasp what the issue really was. the true issue deals with a limitation with base16 if i understand correctly.

and although i personally like the looks of zellij with this PR, that makes it a subjective thing that i can easily overwrite in my own config 😁

getting this PR merged is not a hill i will die on, so i'll leave it up to the community and maintainers since i don't believe it really fixes any actual issues 😁