You can edit your site styles on the Styles customisation page (Styles>Custom styles). Customising a site is a complex subject, and there is a separate guide covering all the topics. To download this guide, see the link at the top of the style settings customisation page.
NOTE: We CANNOT offer general support on customising your site. In particular we cannot offer advice or support on the creation of background images or how to match colours. If in doubt, use the default setting for the parameter in question. If you experience an error or clearly incorrect formatting on your site, let us know via a support ticket, but if you're having trouble understanding what a feature does, please refer to the customisation guide or the online help and try experimenting with different settings.
To see how the system works, open the Basics branch of the tree. Select a setting to edit (e.g. page width) and click the edit link to the right of the page.
Adjust the setting as required and click update. Preview your site, and then if you want to keep the change, YOU MUST click save settings on the custom styles page.