The only way to currently change a forum avatar is through Gravatar.com. The email address used for your Userstyles.org account must be one of the email addresses that is registered to your Gravatar account.
Adding a CSS Rule ?
I'm new to creating my own userstyles and unexperienced with css.
I added a CSS rule with the Firefox developer tools (inspector, Rules tab) and it worked easily.
To be more precise I clicked on the + button and added 2 lines in there.
Now I would like this new rule to stay when I refresh the page, go in subdirectories of the domain or restart the browser so I thought creating a userstyle would be the way to go. Am I right on this ?
If so, what's the process for simply adding a CSS rule ?