Guidelines for editing the CSS testsuite with github

Hi,

I would like to make some edits in the CSS Background and Borders 
testsuite, to correct tests which are wrong or are missing ref file image.

I am new to Github and the CSS testsuite.

Apart from the following general ressources [1] , [2] are there some 
guidelines/tutorial for editing the CSS testsuite with github?
Where are the repos of the tests ?

Thanks for your help,

Thierry

[1] Introduction to GitHub
http://web-platform-tests.org/appendix/github-intro.html

[2] web-platform-tests documentation
http://web-platform-tests.org/