How do I make a HubSpot theme the default for new pages?

Start creating a website or landing page, open the theme selection screen, and choose Set as active theme on the theme you want. For website pages, you can also use the Theme menu while choosing a template, select Change theme, and set the preferred theme as active. Its templates will then appear as the default options for newly created pages. This does not switch existing pages to the new theme; those pages keep their current templates until you change them.