Stop writing CSS for one smaller heading.

Not built in. Squarespace 7.1 stops at Heading 4 and Paragraph 3, and no setting adds more. TextLevels adds Heading 5, Heading 6, Paragraph 4 and Paragraph 5 to the same text menu — and on the published page Heading 5 is a real <h5> and Heading 6 a real <h6>, so search engines and screen readers see the outline your readers see.

Yes — that is exactly what TextLevels does. Paste four lines into Settings → Advanced → Code Injection → Header, once. Everything after that is the text menu and your font panel, and the plugin never reads or writes your Custom CSS.

Automatically. Squarespace sets font, weight and spacing once for all heading levels, and the new levels inherit all of it — so they always match the rest of your type. Size is the per-level setting, with sliders in Site Styles right under the ones you already have.

Any Squarespace 7.1 template. The plugin reads your template's own Heading 4 size and continues the ladder one clean step down, so it starts sensible everywhere. You need Squarespace 7.1 (not 7.0), a Core plan or above, and an English-language admin — the docs have the detail.

Your content is never at risk. Each level is stored as an ordinary paragraph with a class on it, so removing the four lines leaves your text exactly where it was — the four levels simply revert to ordinary paragraphs at their default size. And every purchase has a 30-day money-back guarantee.