how do you make a custom theme GDF compliant ?

what if someone wants to use XYZ theme they find on, say, themeforest

how can they make that theme compliant with GDF so that their site can totally look like the XYZ theme they like?

just wondering what pieces and such need to be adjusted for total compliance for a seemless look using a custom theme (as a child theme)

for example, it sounds like the following was all that was needed to get the Avada theme compliant:

https://wpgeodirectory.com/docs/avada-compatibility-header-php/

thanks