How can you change SharePoint online "Change the look" using custom code instead on out of the box feature option

Hi @Mirza Hassan Baig ,

Welcome to Q&A forum!

Unfortunately, currently for the SharePoint Online modern experience, you cannot use JavaScript.

This extensibility option is only available for classic SharePoint experiences. You cannot use this option with modern experiences in SharePoint Online, like with communication sites. Notice also that you should not take a dependency on the html page structure or on the out-of-the-box CSS style names as these might be adjusted without a notice.

For your reference:

https://learn.microsoft.com/en-us/sharepoint/dev/solution-guidance/customize-your-sharepoint-site-ui-by-using-javascript

You can try to use SPFX. For more details, please refer to:

https://learn.microsoft.com/en-us/sharepoint/dev/spfx/use-theme-colors-in-your-customizations

Good day!

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