function hasTheme(expression) {
      return angular.isDefined($mdTheming.THEMES[expression.split('-')[0]]);
    }
Check if expression has defined theme
For instance:
  'myTheme-primary' => true
  'red-800' => false
@param {string} expression color expression like 'red-800', 'red-A200-0.3',
  'myTheme-primary', or 'myTheme-primary-400'
@return {boolean} true if the expression has a theme part, false otherwise.