feat: Add themes and allow cycling thru them

[BURG3R5]

Description

Fixes #10

As I mentioned in #10, support for additional themes will be a really nice QoL upgrade... so I implemented it.

I've changed the theme.toggle shortcut to theme.cycle which steps through an arbitrary list of themes. I've also added four themes: black, red, green and blue. I've updated the relevant test.

Additional context

(Kindly ignore the sliver of grey in the bottom-right corner.)

• Black:

• Red:

• Green:

• Blue:

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[FPurchess]

Great work! Thanks for your contribution

[BURG3R5]

@FPurchess If it's alright with you, can you approve the workflows and merge the PR?