Test and improve docs for plugin styling

[microbit-matt-hillsdon]

Follow-up from this comment thread.

The CSS has got quite subtle, see e.g. #648.

Let's add some improved comments and start to test key styles are applied in the various focus scenarios (maybe via getComputedStyle(), other suggestions or existing examples of such tests in Blockly welcome).

Note the in-progress #637 - it's probably worth waiting for that.

[microbit-matt-hillsdon]

Comments were added back on #643 so #667 just adds test coverage.