SharePoint Designer Settings can be configured per web application level. Configuration is simply straightforward in SharePoint online environment.
These are the list of options available on the settings page. Just a tick mark would enable respective settings for you.
- Allow Site Owners and Designers to use SharePoint Designer in this SiteCollection
- Allow Site Owners and Designers to Detach Pages from the Site Definition
- Allow Site Owners and Designers to Customize Master Pages and PageLayouts
- Allow Site Owners and Designers to See the Hidden URL structure of their Web Site
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