[A11y]Fix some minor text contrast issues. (#8164)

* Darken link colour with darker background.
* Fix contrast on reset for advanced search

Author: ryuyu

src/Bootstrap/dist/css/bootstrap-theme.css

@@ -2,6 +2,10 @@
 
 .bg-variant(@color) {
   background-color: @color;
+  & a {
+    color: darken(@link-color, 10%);
+  }
+
   a&:hover,
   a&:focus {
     background-color: darken(@color, 10%);

src/Bootstrap/dist/css/bootstrap.css

@@ -83,6 +83,10 @@
       margin-left: 10px;
       margin-right: 10px;
     }
+
+    #reset-advanced-search {
+      color: darken(@link-color, 10%);
+    }
   }
 
   @media (min-width: @screen-sm) {

src/Bootstrap/less/mixins/background-variant.less

@@ -956,7 +956,7 @@
                             linkText: "Reflow package",
                             actionName: "Reflow",
                             controllerName: "Packages",
-                            role: string.Empty,
+                            role: "button",
                             formValues: new Dictionary<string, string>
                             {
                                 { "id", Model.Id },