Bug 29036: (follow-up) Improve the use of variables to keep colors consistent

Bug 29036: (follow-up) Improve the use of variables to keep colors consistent

This patch modifies the OPAC SCSS so that a "base theme color" variable
is defined which can be used to color button backgrounds and similar
elements. The patch also moves some other colors into variables and
removes some unused CSS.

The Koha logo which is displayed in the upper left corner of the OPAC
has been modified to use the same color.

To test, apply the patch and repeat the previous test plan.

Signed-off-by: jeremy breuillard <jeremy.breuillard@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>