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Hi Anna,
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.
If you mean the several pre-set styles don't meet your requirement and you want to add custom sytles, there is no out of box solution to change the several pre-set paragraph/heading styles in SharePoint Online without changing or adding CSS files. If we want to add some custom styles to SharePoint Online, we need to modify or add CSS files. Below are two articles for your reference:
Use CSS to brand SharePoint pages
How to add custom styles to the ribbon in SharePoint 2013
Disclaimer: Microsoft provides no assurances and/or warranties, implied or otherwise, and is not responsible for the information you receive from the third-party linked sites or any support related to technology.
**Note:**If we want to custom the master page of SharePoint Online, we also need to modify the code of master page with SharePoint Designer or add a CSS file as master page.
We understand it’s more convenient if there is out of box solution for adding custom styles to SharePoint Online. It is suggested you submit the requirement via Uservoice. Let the related team hear your voice. This is the best way to influence our products. Thanks for your understanding.
Meanwhile, you may also monitor this thread to check if other community members have good suggestions.
@Other community members are welcome to share your experience here.
Best regards,
Tim