Use `print-color-adjust` property in the theme scaffold

[yhatt]

An upcoming Chrome 136 will be implemented an unprefixed print-color-adjust property.
https://developer.chrome.com/blog/chrome-136-beta#unprefixed_print-color-adjust

Marpit framework is still using -webkit-print-color-adjust and color-adjust (A legacy alternative for Firefox).

marpit/src/postcss/printable.js

Line 44 in f36aee5

-webkit-print-color-adjust: exact !important;

-webkit-print-color-adjust will not be removed in an upcoming Chrome, but it is better to prepare against removing legacy properties in the future, by defining unprefixed property together with legacies.