You need Prewikka 5.1.0 or higher.
The git repo for Prewikka apps is available on GitHub and can be cloned like this:
git clone https://github.com/Prelude-SIEM-Contrib/prewikka-apps-customcss.git
Once the source code downloaded:
python setup.py install
Then restart Prewikka.
The code is released under the GPLv2 license. See the COPYING file.
Please feel free to propose your own Prewikka apps by submitting a pull request.