Add Dara theme docs

[PatriciaJacob]

Motivation and Context

Docs didn't have an easy visual representation of the theme colors available

Implementation Description

Updated the docs and added two images with the current pre-built themes, also updated the plotting theme section because it is not only Bokeh that can have one set.

Any new dependencies Introduced

None

How Has This Been Tested?

Tested that the code snippets were reasonable on a test app

PR Checklist:

• I have implemented all requirements? (see JIRA, project documentation).
• I am not affecting someone else's work, If I am, they are included as a reviewer.
• I have added relevant tests (unit, integration or regression).
• I have added comments to all the bits that are hard to follow.
• I have added/updated Documentation.
• I have updated the appropriate changelog with a line for my changes.

Screenshots (if appropriate):

[krzysztof-causalens]

Looks great, thanks for the first time contribution 😉