Moving color contrast discussion from https://bugs.launchpad.net/evergreen/+bug/1960526/comments/8 since its parent bug is now closed, and this encompasses buttons as well as links.
Several of Bootstrap 4's default colors don't pass WCAG level AA tests, including the primary blue (#007bff) used for both links and buttons throughout the staff interface.
The link color proposed in #1960526 does pass level AAA checks on the main contrast checking tool, but doesn't pass the link contrast checker (https://webaim.org/resources/linkcontrastchecker/) because it lacks contrast with the body text color. Note that we need to test with #f7f7f7 as the background color, since this is used in tabs and tables.
Level AAA compliance is complicated, and introduces problems for people who don't have low vision. I think we should consider a separate high-contrast mode that could be set in user preferences for those who need it, and stick to Level AA compliance otherwise
I've put together a quick comparison of all four link and button colors that are currently failing automated accessibility tests: https://codepen.io/stephanieleary/pen/poVaPYq
The second set shows the minimum change needed to pass the Level AA tests, and the third set shows the nearest Bootstrap 5 tints (https://getbootstrap.com/docs/5.0/customize/color/#all-colors). I would recommend the Bootstrap 5 colors even though they are ever so slightly darker, since sticking to the defined palette will help us stay consistent.
I also threw in a button using the Evergreen brand color in the second set, but I'm leaning against adopting that color for green/success because it's so similar to the cyan/info color. If we want to use the Evergreen shade for buttons, we'll need to consider choosing a different color for btn-info.
If you agree on the Bootstrap 5 scheme, here's a patch: https://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/sleary/a11y_btn
It does not include the optional text shadows; I plan to do a separate patch later to add subtle shadows to several parts of the interface where depth can be hard to perceive, like nav menus that drop down over dense text tables.
Noting for anybody reading this that the Angular and ng-bootstrap upgrade proposed in bug 1982376 would force the issue of also upgrading to Bootstrap 5.