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To edit your global CSS stylesheet:

Choose Administration menu , then General Configuration > Stylesheet.
Choose Edit.
Paste your custom CSS into the text field.
Choose Save.

Note:

The new CSS will be visible across all spaces, provided they do not define their own custom stylesheet and are not using a theme. This CSS will also overwrite all styles defined in custom global themes.
You may be able to add CSS to your site by choosing Custom HTML in the administration section, and adding your CSS definitions to the HEAD or BODY of the page. You should only use this option if you cannot achieve the desired results via the global stylesheet.

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