Each Page of your site has its own Page Theme. To edit a Page Theme, first browse to the Page, then right-click anywhere on the page and click Edit Page.
Align Content Grid
The top setting on the Page Theme editor allows you to choose how the main content grid for the page will be aligned.
If the grid is 100% width, this will have no noticeable effect. But if you resize your content grid to be less than 100% wide, or if your grid is pixel based, this setting may be useful to you. As an example, let's say we are creating a page whose content is fixed width at 900px. We can either choose to align the content to the left hand side of the browser or to center it so that it is down the middle of the page.
Template / Content Layout
Next we can choose the Template that this page will use. Each Page can use the same template but you may also use different templates for different pages. Select the template based on wether or not you want to display a header and sidebar.
On the Theme tab you can override values that are set in the Site Theme Settings. For instance, if you want one Page to have different content background color than the rest of the site, you can add
--body-bg-color on this tab with your page-specific value.
And here's the result: