Loading custom themes, standard/recommend method?

When loading custom themes made by the user & stored somewhere for example local storage, I assume the flow is: 1-Read theme data from storage 2-Flatten theme & generate CSS text 3-Dynamically inject it as a style to the document, os use cssom to add sheets 4-Use [data-theme="custom1...n"] to switch themes Or Have a single placeholder [data-theme="slot"]{} In your static CSS, just read the user selected th