Theming: Improve color blocks and theme section defaults

[diondiondion]

Some aspects of our theming system could use some streamlining to make it both easier to use and more flexible.

Issues:

1. Button colours are currently global and separate from both Theme Sections and Color Blocks. They're simpler than a Theme Section, and slightly more complex than a Color Block (giving separate control over text and background colour). In a way, they're an escape hatch I introduced because neither concept was fitting 100%.
2. Neither Color Blocks nor Buttons can be customised per Theme Section. Since Buttons are global, they may clash with some of the colours set for a theme section. Ironically, the negative example in the Storybook theming explainer can still easily happen in base5-ui. There needs to be a theme-level escape hatch for that.
3. Theme Sections often share similar variables. Especially the fields links, highlight, textDimStrength, shadeStrength, and lineStrength are often the same across multiple Theme Sections.

Approach:

• Allow specifying a default Theme Section sections.default which will be extended when variables are missing from any other section
• Move default Color Blocks from globals.colorBlocks into sections.default.colorBlocks; then allow overriding them in other sections
• Allow defining Color Blocks in more detail. Currently, they extend a Theme Section named colorBlock, replacing its text, links, and background fields, without precise control over how that's done. It should be possible to instead of assigning a simple colour string, assign an object that overrides the default Theme Section (e.g. instead of positive: 'green', allow positive: {text: '#fff', background: 'green'}). Again, missing fields would be taken from the default.
• With the aforementioned update, buttons could then be turned from special global variables to simple Color Blocks, e.g. button-default, button-primary, and button-important.

Implementation:

• All of this needs to be done in a backwards-compatible way, we can't break existing themes
• Investigate whether the colorBlock section we're currently using is still necessary when we introduce a default Theme Section The colorBlock section has been removed in the meantime ✅