Hi @Khushi ,
To create a communication site, you need to be a SharePoint administrator. Here's an article on creating communication sites in SharePoint for you to refer to: Create a communication site in SharePoint.
To design a communication site (use a custom theme or using powershell scripts), you need to be a site owner or a site member with Design or Full Control permission. For more detail information,please refer to:Permission levels in SharePoint
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