fix(theming): fix CSS with nested rules parsing when registering a theme

Replace the regex used to split the CSS string with a function that takes into account nested rules.

Fixes #9869

Author: natete

src/core/services/theming/theming.spec.js

@@ -965,4 +965,4 @@ describe('$mdThemeProvider with custom styles that include multiple nested rules
     // Find the string containing nested rules in the head tag.
     expect(document.head.innerHTML).toContain(compiledStyles);
   });
-});
+});