How to change the modern theme on classic sites without PowerShell

Once a wise man cited a wiser woman with the following statement: “My mom always said life was like a box of chocolates. You never know what you’re gonna get. – Forest Gump”. This quote can be applied to SharePoint in Office 365 nowadays too. When you have a provisioned classic site or even a modern site, then you never know what primary colour you get as your default theme.
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