This forum is only for questions or discussions about working with the mojoPortal source code in Visual Studio, obtaining the source code from the repository, developing custom features, etc. If your question is not along these lines this is not the right forum. Please try to post your question in the appropriate forum.
Please do not post questions about design, CSS, or skinning here. Use the Help With Skins Forum for those questions.
You can monitor commits to the repository from this page. We also recommend developers to subscribe to email notifications in the developer forum as occasionally important things are announced.
Before posting questions here you might want to review the developer documentation.
Which style sheet class applied to master page by default...
There a lot of CSS files used in mojoPortal, and they are all combined and minified using the CSSHandler. I recommend you check out the Skinning documentation: http://www.mojoportal.com/skinning-overview.aspx.