Fix print output inheriting screen theme colors

[giri256]

Fixes #1934.

Summary

• reset print-only content variables to mdBook's existing light palette
• keep all selected screen themes unchanged
• cover page, text, quotes, tables, keyboard keys, footnotes, and callouts through the shared variables

Verification

• Chrome print-media computed-style matrix for ayu, coal, light, navy, rust, and no-JavaScript navy
• navy screen-media negative control
• cargo test -p mdbook-html --lib
• cargo fmt --check
• git diff --check

[GuillaumeGomez]

Instead of redefining the variables, would be simpler to find a way to reuse the light variables from variables.css.