add example of replicating original site themes in bootswatch

[machow]

This PR illustrates how you can set bootswatch variables to edit a theme. I realized that a lot of the CSS I took from siuba.org does funky things once you start tweaking styles, so removed it.

Some key pieces:

• changed styles.css to scss, so it can set bootswatch variables
• added link to darkly theme scss variables (theme preview here)
• Experimented with recreating original event site's theme of dark purple with white text.

Here's a screenshot:

I'm a theming barbarian, and this was more me getting a feel on how quarto allows theming. Feel free to crib anything into other PRs or punt this PR :).

[travissouthard]

Closing and will add these changes to #9