make abbr color inside bibliography a variable (alshedivat#3017)

Fix alshedivat#3012 (make abbr color
inside bibliography a variable)

Author: diaoenmao

_sass/_base.scss

@@ -739,6 +739,7 @@ footer.sticky-bottom {
           display: inline-block;
           background-color: var(--global-theme-color);
           margin-bottom: 0.5rem;
+          color: var(--global-card-bg-color) !important;
 
           a {
             color: white;