Contrast theme extends dark theme

[lindapaiste]

The dark and contrast themes have a lot of the same properties. This PR makes it so that the shared settings only need to be specified once, as contrast will inherit any unspecified properties from dark.

Changes:

• Add a utility function extend to merge themes.
• Delete properties of contrast which are duplicates of dark.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• [] is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

Thanks for working on this!