You can manipulate the CSS for each individual Theme CSS. The code is available for editing from within the Code tab.
To edit the code for a Theme CSS:
Open the Design tab and click Code.
Scroll down to view the Theme CSS options and select the one you want to edit.
The design code window for that Theme CSS opens.
Click the Style Sheet tab.
Immediately above the menu tabs is the Expand Editor icon which allows you to view full screen mode.
Modify the code, as required, or click Restore to return to the last version of the code saved by the system in Layout manager, (losing all code changes made since then).
You can click the Preview button to see how the changes will appear in your web store before going live.
