As others replied already - PeppyMeter was written in Python using Pygame module/library. It’s so called rich client which is usually running on displays connected to you computer not remote browsers. It cannot run in a browser directly like Javascript. Therefore you cannot use CSS3 (used for styling web pages) with PeppyMeter UI.
But besides of displays connected to you computer PeppyMeter can output volume signal to other devices as well. Those devices should be connected to your computer (e.g. Raspberry Pi) using such connections/protocols as I2C, Serial interface, PWM and HTTP:
Outputs · project-owner/PeppyMeter.doc Wiki · GitHub
Using HTTP protocol PeppyMeter can send a volume signals to browsers over your local network. Though in this case it’s your responsibility to implement the whole UI. While doing that you can use CSS3 or any other web technology which you know/prefer.
Best regards