I found how to launch Firefox using this line of code with custom theme:
bash -c 'GTK_THEME="Arc-Darker-GalliumOS" firefox %u'
The thing is that when I open a link from an application it uses the default theme, which doesn't work well with Firefox.
So I opened up Preferred Applications and set as default web browser this line of code.
When I tested that, instead of opening the URL, it just went to
How can I fix that?