Fix toolbar visibility after Firefox v137 update

[Sneaky1000]

Fixes #242

This simple fix forces the z-index set in shy-global.css to override the value set by Firefox in version 137.

[Sneaky1000]

My ShyFox is a bit more customized, but it looks like this after the patch:

[Sneaky1000]

@Naezr I want to add that I'd love to help maintain the repo while you are inactive. This is by far my favorite Firefox theme and keeping it up-to-date is something I'm very much interested in. Unfortunately, Firefox frequently breaks the theme for everyone, so please let me know if there is any way to make this happen. Thanks!

[sleepymincy]

@Sneaky1000 seems like the original author somehow MIA even if the contributions activity says otherwise? I'd recommend you to just fork it at this point, I'll be happy to follow your updates and I'm sure other users will be happy too. If Naezr wakes up at some point and agrees to make you a maintainer you can just merge your changes and/or archive the fork later.