This is usually private, and only accessable to you.
It's usually located at <username>/.config.
Clone it onto your local system using Git, and cd into it.
The command to clone the repo is git clone <remote url>.
You can find the remote URL for your repository in the "Information" button.
Once clicked, a popup will appear and a textbox labled "Clone over HTTP" will
be in that popup. The URL in there is your remote URL.
cd changes your working directory.
The directory your cloned repo should be in is .config
Copy the Theme CSS of your chosen them into a file named theme.css within this directory.
The Theme CSS can be found by clicking "Theme CSS" link under the theme you chose.
Commit back to your .config repository with the new file.
You can do git add .,
git commit -m "Added theme.css", then
git push.
Congratulations, you can now go to your chosen instance of Roundabout with your
new theme installed!