It is not easy to know how to answer when you don't show your current CSS and HTML. For starters, do you mean Capitalize (in non-CSS contexts this is often called Title Case) or do you mean UPPERCASE?
Note that Capitalize behaves differently from the other two:
Puts the first character of each word in uppercase; other characters are unaffected.
In other words, "capitalize" will not change an original capital letter into lower-case; only "lowercase" will do that. No, you can't cheat by wrapping a "capitalize" around a "lowercase" (I tried).
It may help to backtrack and explain what you're trying to do. Is your original text in ALL CAPITALS? How did it get that way? If you're dealing with user input, you may be able to fake-up "Sentence case" by putting a "capitalize" tag around the first letter and a "lowercase" around the rest. But that's pretty clunky.