Learn English – a word that describes the class that “background” and “foreground” are members of

A few ideas: IVariable (don't think it will suit your needs here due to "variable" ambiguity), IAdjustable, IModifiable, IMutable.

Consider art-lover. It is listed in OED also and the earliest citation is from 1847:

Last year was a surprise to every art lover for the crowd of excellence it produced.

_{Fine Arts' Jrnl. 8 May 422/2}

The painter Renoir once said: (mentioned in the book The Hidden Renoir)

The art lover is the one to whom the medals should be given.

Dilettante originally meant art-lover but it has negative connotations today. Vocabulary.com explains as below:

The meaning of dilettante has changed since it was borrowed from the Italian in the mid 1700s. Originally, it meant "lover of the arts," but began to take on a negative slant as the idea of doing something as a professional took hold strongly during the 18th century. A dilettante was a mere lover of art as opposed to one who did it professionally. Today, the word implies you're pretending to be more of an artist than you're interested in or capable of being, so if you call your friend who likes to paint a dilettante, it's like you're calling him or her a poser.