In the first part of your sentence, I would make it sound more natural by deleting a single word: "los". The result would be:
Porque Starry Night tiene colores oscuros…
For the second part, if you insist on using the word "belleza", I would say:
…además tiene una gran belleza.
I don't know if this was intentional or not, but it's a good sign you avoided using the word "bello" because it sounds odd to a lot of people. It's not a mistake but "(posee) belleza", "(es) lindo", "bonito", "hermoso", "divino" are more common in my experience. In the last two cases ("hermoso" and "divino") the adverb "muy" may be redundant.
What throws me off a little bit is the repetition of the verb "tener" ("tiene"). So my correction would go even further:
Porque Starry Night tiene colores oscuros, además es de una gran belleza.
But another thing that throws me off is not about your Spanish but about your original meaning (even in English). The choice of "además" (or "también") sounds weird to me because it sort of implies that colors and beauty are independent and I don't think this is correct when talking about an artwork. I feel the relation between both qualities (dark colors and beauty) would be more of the cause-effect type and not an addition. For example, Starry Night may be beautiful because of the dark colors but I can't see how an artwork can have dark colors independently of its beauty (like using "además" implies). It's like saying: "Starry Night has dark colors, moreover it has a lot of beauty." In that sense, in a written text I would go for a semicolon, so my definitive result would be:
Porque Starry Night tiene colores oscuros; es de una gran belleza.
Even simpler and more natural:
Porque Starry Night tiene colores oscuros y es de una gran belleza.