The look of the web interface is based on the Bootstrap theme.
You can find Bootstrap 4 themes at https://bootswatch.com/.
From there you can download a bootstrap.min.css file and replace the existing one located in the installation folder at inc/css/.
Currently the web interface uses Bootstrap 4. Previous versions of litebans-php used Bootstrap 3. These Bootstrap versions are not forward-compatible nor reverse-compatible, so don't use old themes or everything will break.