feat: better management of themes and updated default theme

[albanm]

• Remove the inlined default theme from api/config/default.cjs and source it from @data-fair/lib-common-types (defaultTheme), merged under any user/env
  overrides in api/src/config.ts.
• Use a default theme with 2 main bases (blue and orange) and analogous colors inside these.
• Manage readability of tonal contents