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If you want to change the Paragraph and Heading on the SharePoint Online page, you can click the setting gear>Edit page>Format TEXT>choose a Paragraph/Heading you want in the Styles section.
Is it possible to modify a built in style?
Modify the built-in styles Fortunately, you can modify built-ins in one of two ways. First, the long way: Click the Home tab, right-click the style you want to modify in the Style Gallery, and then choose Modify. In the Modify Style dialog, change style attributes.
Applying custom CSS to a SharePoint page. Apply CSS to a SharePoint site by using an external style sheet and including a reference to the style sheet in the tag inside the tags of the SharePoint page. The following code example shows how to upload custom CSS to the asset library, apply a reference to the CSS URL…
How to create a custom style sheet for SharePoint?
To create a custom style sheet for SharePoint Make a copy of corev15.css and name it contosov15.css. Open the relevant masterpage (in this example, contoso.masterpage) and modify the CssRegistration line to point to contosov15.css from corev15.css, as shown in the following example.
How does CSS change the look of a SharePoint page?
The comments tell SharePoint to change the attribute of the CSS that immediately follows the comment. Applying a composed look might change many of the default colors, fonts, and background images that are applied, and subsequently update the settings in corev15.css.
Each .master file refers to the corev15.css file, which is built from a variety of .css files delivered with SharePoint out-of-the-box. All default master pages use corev15.css when processing styles, and rely on the CSS cascade and CSS file sharing to resolve which styles are applied to specific controls and elements in regions of a page.