function to generate values of a javascript object

The following code works but I am wondering if it can be improved both readability and performance.

I am using jquery and underscore.

Any suggestions, comments or feedbacks will be appreciate.
thanks.

var getThemeName = function (theme_name, transparency) {
    if (theme_name === 'light') {
        if (transparency) {
            return translator.get('messages:theme.info.light_with_transparency', {
                percentage: transparency
            });
        }
        return translator.get('messages:theme.info.light');
    } else if (theme_name === 'dark') {
        if (transparency) {
            return translator.get('messages:theme.info.dark_with_transparency', {
                percentage: transparency
            });
        }
        return translator.get('messages:theme.info.dark');
    }
};

var THEME_ITEMS = {
    themeLight: getThemeName('light'),
    themeLightTransparency75: getThemeName('light', 75),
    themeLightTransparency50: getThemeName('light', 50),
    themeLightTransparency25: getThemeName('light', 25),
    themeDark: getThemeName('dark'),
    themeDarkTransparency75: getThemeName('dark', 75),
    themeDarkTransparency50: getThemeName('dark', 50),
    themeDarkTransparency25: getThemeName('dark', 25)
};

If you keep the naming consistent, you could probably do something like:

function getThemeName(themeName, transparency) {
  var name = "messages:theme.info." + themeName;
  if( transparency ) {
    name += "_with_transparency";
    return translator.get(name, { percentage: transparency });
  } else {
    return translator.get(name);
  }
}

StackExchange Code Review Q#17640, answer score: 4