Added Basic Dark Mode Support That Utilizes Toggle and Local Storage

[xkjjx]

TODO

• run SimpleCov tests to ensure they are passing (and possibly add new tests)

Changes

• Addresses and closes Dark Mode for Output Files #147
• makes CSS formatting neater by introducing variables defined at the top of the file
• removed irrelevant private tag const variable for CONTENT_TYPES by making it non-private (const variables are public even if placed under the private tag)

Review Tips

Check out sample test results formatted using old styles HERE and with my new styles HERE.

I recommend not looking into the individual commits and checking out the PR as a whole. I also recommend squash merging.