fix(autocomplete): align highlight style with spec (#11244)

fix a11y issue where difference between highlighted text is too small

Fixes #10060

Author: Splaktar

src/components/autocomplete/autocomplete-theme.scss

@@ -21,9 +21,6 @@ md-autocomplete.md-THEME_NAME-theme {
   background: '{{background-hue-1}}';
   li {
     color: '{{foreground-1}}';
-    .highlight {
-      color: '{{background-600}}';
-    }
     &:hover,
     &.selected {
       background: '{{background-500-0.18}}';

src/components/autocomplete/autocomplete.scss

@@ -188,6 +188,10 @@ md-autocomplete {
   // Expand the virtualRepeatContainer as much as the max-height from the JavaScript allows.
   // This is necessary for the virtualRepeatContainer to be able to grow back.
   height: 100%;
+
+  .highlight {
+    font-weight: bold;
+  }
 }
 
 .md-virtual-repeat-container.md-not-found {