Support for userchrome manager

[111ebobo]

Before sending your suggestion

• My version of Firefox is up to date (for 115, this means being on at least 115.16.1)
• My copy of Geckium is up to date (no update option in the hamburger/spanner menu)

What browser are you running Geckium on?

Nocture\Marble

What Operating System are you running Geckium on?

Windows 10 21h2 LTSC

What would you like to suggest for Geckium?

Please add support for userchrome manager by travis (https://github.com/travy-patty/userchrome-manager), it'll help people to switch this theme to others more conveniently

Additional Context

No response