API Newbie: Can I alter CSS?

The idea is simple: give the players two buttons to change the interface. The main thing I want to alter are player webcams, (size & position). How can I archieve this using the API?

You can't.

You can't with the API.  You can can use a browser extension like Stylish to override the CSS with your own styles. 

As Aaron says, I use Stylish to do a few minor tweaks and the last time I looked at the CSS (albeit it may have changed) it was possible to relocate the webcams (I moved them down the right hand side). If you install stylish, write a CSS mod for the page and then you should be able to send it to your players and walk them through enabling it.

Ty for your replys. I already did some hacks using direct css in the developer console to alter the webcam appearance, but I need to make it the easiest for the players, since they change a lot from week to week, (Roleplaying community). The perfect approach is API, (seems cant), Iframe + css editing (cant either), or other javascript way to make it work in a single page. I will have to research for the last option.

As an alternative, if you don't want to use plugins then try having a look at&nbsp; <a href="https://app.roll20.net/forum/post/1174957/slug%7D" rel="nofollow">https://app.roll20.net/forum/post/1174957/slug%7D</a> as a starting point. A javascript 'bookmarklet' which your players should just be able to add this to their bookmarks bar and just click it to update the CSS of the page. Even if you don't know JavaScript, if you can workout and amend the CSS via the developer console then you should be able to work out how to do something similar using this.