fix: theme must own both light and dark values for every primitive it sets

[LukasHirt]

Fixes two regressions from the previous theme fix, both root-caused and verified in an actual browser this time:

1. Cross-layer priority ignores selector specificity entirely, so once this file's dark-mode values sat in a higher-priority layer than Starlight's own, they permanently beat Starlight's lower-priority light-mode values for every primitive touched here - breaking the light/dark switch again, just for surfaces instead of accent. Fixed by explicitly setting both themes' values for every primitive this file touches, rather than relying on Starlight's own conditional logic to fill in one theme.
2. A color-mix() call had its percentages backwards (35% orange + 65% white reads as pale pink, not a lighter orange). Flipped to 80% orange.