Add support for themes

[pumbas600]

Related Problem

Currently, every user has to manually customise the look of their graph. This is not too bad currently, but as more styling options get added this becomes more and more challenging and makes it less accessible for new users.

Requested Feature

Add a theme option that maps to a predefined style. This should be used to set the base options which can then be overridden by any other style query params.

This should probably be contained in a single file to make it easier for others to add new themes.

This will also require a new dropdown to be added to the playground.