css adjustment to make buttons indepedent of other elements

[Rogan Hamby]

One challenge of customizing the opac for new libraries is that buttons are dependent on other color elements like primary and primary fade.

A few things that need to be done:

adding colors to colors.tt2 to distinguish button background (and hover), button text (and hover) and button border. Some of these already exist, some may need renamed and some need to be created

adjusting style.css.tt2 for this, specifically the .opac-button and .opac-button:hover sections though to have the hover color take precedence over the header style for the my account button it also needs an important tag, e.g. color: [% css_colors.button_text_hover %] !important;

As part of this I'm also suggesting taking out the opac-button-header style in the topnav.tt2 and style sheet so that all buttons can act consistently.