Dark theme: MDC primary + fonts migrations.

[isoos]

• Dark mode for pub.dev #4416
• Merged two font definitions where the only difference was an extra preference for "Google Sans Text" over "Google Sans".
• Note: deployed to staging (ongoing)
• Note: it looks like the material components we are using are using 30-40% of the predefined color variables, otherwise they are using baked-in values. It will take a bit more effort than expected to migrate them.