Small fixes

[selfthinker]

A couple of small fixes following recent PRs.

• Change style.ini placeholders to LESS syntax: I should really have done that as part of the move to LESS (Use LESS instead of CSS #166). And it's also required to it possible to styles PDFs with the screen styles (use default styles in pdf.less #167).
• Make '.dokuwiki' scope more consistent in shared print and pdf styles: The same, should have been part of Use LESS instead of CSS #166 and is required fro use default styles in pdf.less #167.
• Change code in boxes and notes to normal text: When forcing content colour of safety notes (Force content colour of safety notes #169), the colour of the code blocks in some of them were white on a light grey background. Instead of fixing that one case (out of probably many), I opted for adding a note saying safety boxes work best with simple markup and changing the code block to normal text. So that it doesn't look too weird, I also changed the code blocks of the boxes above to be normal text.