While Firefox extensions are a nice to have it's better to provide ways to show own themes. And the best way to customize is via extensions. That would also be a solution for bug 61205.
*** Bug 121163 has been marked as a duplicate of this bug. ***
Sahil, resolve as fixed?
https://gerrit.libreoffice.org/c/core/+/174835 patch adds UI for the new Appearance tab which allows theme customization. It has a `more themes` button to install themes via extension.