Establish brand colors and organize CSS variables etc

[jgaehring]

See below:

• Establish consistent brand colors and theme  the-runrig-plan#5
• Add Pico CSS framework #23 (comment)

We should be able to adopt the default theme colors from vitepress@1.0.0-beta.3, which is what we originally based the runrig.org color theme upon, then adapted it to the EWS here. We can probably even reuse some of those same CSS declarations from VitePress. As I noted in #23, it might be best to wait for Pico 2.0.0 before doing any of this, though, so we can take advantage of their new API for customizing CSS Variables.