Navigate to the gallery for which you want to create a CSS (could also be your root gallery) and click on 'edit this galery' link on top. In the stylesheet box, click on link that says 'edit stylesheets'

In the resultant screen, either select one of the existing CSS' in the table to edit it or enter the name of a new stylesheet if you want to start from scratch (here we call it my_new_stylesheet)

Click on 'New CSS' button

PBase now creates a template for the new CSS and you can add your CSS here

Once you are done, click on 'Update CSS' button at the top

Once you are done creating the stylesheet, go to the gallery where you want this new stylesheet used and in the stylesheet drop down list, you should now see the newly created CSS - select it and click on 'Update Gallery'.

It's that simple !!