The size, colour and weight of the headings used in LibGuides is determined by our CSS stylesheets. What you see when editing is not what an end-user will see when viewing the guide. Use the preview button (the little eye icon in the top right to see what your guide truly looks like)
The title of your guide is a Heading 1; the titles of the pages and boxes are Heading 2. Headings used within the boxes should start with Heading 3. Headings are used to help users understand the structure of your content. Proper use of headings really helps screen-reader users understand the organization of the content.