Bug 26962: Make print.css pure black

Bug 26962: Make print.css pure black

Test plan:
1. Generate some different notice/slips like ISSUESLIP, HOLD_SLIP, any slip you like
2. Inspect and see that body, p tags and see that the computed color is #333333 or #000066.
3. Apply patch
4. The computed style for color should not be #00000
5. This prints much better, especially on thermal/receipt printers

Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>