The Site Style Editor has controls for setting your web-site's fonts and colours. It also controls the width of your site's left and right margins.
Click on the link in the Site Manager.
As you select fonts and colours the sample text of the site style editor will change to demonstrate what your new choices look like. Don't forget to click the Save button or any new choices you have made will be lost.
The widths of the left and right margins can be set using the site style editor. The width values range from thinnest up to widest. There is also the extra setting of hidden which can be used to hide a margin.
Setting a margin width to hidden will result in the margin not being shown, as long as there is no content in the margin. This is discussed more in the help topic Site Border Editor.