Cannot change theme for prism

Great package!

While the `theme.js` file _will_ set visual properties for general styles (e.g. changing the `text` color to `'red'` will change all of the text to the color red), the `prism` properties are ignored.

For example, changing the color for all values to `'muted'` will result in no change to the color of any style -- they'll all remain purple on gray as defined in the package.