This is usually private, and only accessable to you.
It's usually located at <username>/.config.
If you somehow don't have one, you should be able to create it.
Clone it onto your local system using Git,
and cd into it.
The command to clone the repo is git clone
https://<roundabout-instance.example>/git/<your-username>/.config
.
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.
Then you can simply copy and paste the CSS into your theme.css file.
Commit and push your changes to your Roundabout instance.
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!