To make your SiteBuilder "page" a different color from the body background, it is possible to paste the css into the page's HTML head. Click in the page, right click to open the context menu, paste the style in the head section of the context menu options.

Setting the root div width which seems to define the area of the "page" which can be colored still has to be done with a work around. Even if it is specified in the html head css, the root div width is over-ridden by the

tag lower in the page. When you use drag and drop, SiteBuilder automatically sets that tag style to the right edge of the image / text block that is furthest to the right in each page.

To standardize the tag from page to page, insert a background colored image in each page and set the image "X" coordinate. Allow for the width of the plug and any automatic margin, so X would be (width desired, minus plug width and automatic right margin). Don't drag any other image outside that right edge.

A page link to one external style sheet would be more consistent, but the head HTML approach is editable. If you want to change a border, or color, or image name, you can. On the other hand external style sheets are not even visible in SiteBuilder, and the only format it can save is HTML, which automatically inserts site builder html code into any file it saves.

Having the style in each page means any change has to be edited into every page in the site. But perhaps being able to edit the particulars is more important to you than the extra standardization upkeep.

There is also the issue of whether site builder would mess with an external css file when it publishes. Maybe SiteBuilder would leave the file alone. But it might complain about a non standard file, or disable the link to it.

Anyway, here is a second option, totally within SiteBuilder.