chore: bump submodule version (Firefox plugin fix) (#841)

## Description

With recent changes to the Firefox GNOME theme, we just need to override
`gnome-window-background` to match with Gradience's theme instead of
using the default hard-coded value. This bumps the submodules commit to
include the change that has already been merged (see
[this](GradienceTeam/Submodules#4)).

## Type of change

<!-- What type of change does your pull request introduce? Put an `x` in
the appropriate box . -->
- [x] Bugfix (Change which fixes an issue)
- [ ] New feature (Change which adds new functionality)
- [ ] Enhancement (Change which slightly improves existing code)
- [ ] Breaking change (This change will introduce incompatibility with
existing functionality)

## Testing

- [x] I have tested my changes and verified that they work as expected
<!-- Make sure you did this step before marking your PR as ready for
merge. -->

Thanks !