Add Custom CSS

https://wordpress.com/support/editing-css/#tab-site-editor

While most changes can be made with settings in the WordPress editor, you may prefer to use CSS to customize your site if you have experience writing CSS code. This guide will show you how to edit your website with CSS.

Access the CSS Editor

You can add custom CSS to your site from the Styles panel. Follow these steps to add custom CSS:

1. Visit your site’s dashboard.

2. Go to Appearance → Editor.

3. Select “Styles” from the left-side menu.

   • In the editor, you can also select the Styles icon in the upper right corner — the button looks like a black-and-white circle.

4. Scroll to the “Additional CSS” section.

5. Type or paste your CSS into the text box provided.

6. Click the “Save” button at the top right of the screen to save the CSS to your site.

You can preview the impact of your CSS changes on any block type by loading the Style Book. Click “Preview” and select the “Style book” option.

Read more - https://wordpress.com/support/editing-css/#tab-site-editor