Tab Search: Dark mode styles

Adjust Tab search dark mode colors to meet WCAG requirements.
- Selection/Hover colors now match Omnibox colors.
- Fix non-existent variable name

Screenshots:
Before: https://drive.google.com/file/d/1BMIahHv0hNaxuXuAtk228QgGPip9HKJR/view?usp=sharing
After: https://drive.google.com/file/d/16fnTW0bLdwRDp2_VCe3IzuOTsOniGL2k/view?usp=sharing

Bug: 1110109
Change-Id: I087c49a9b98da70ab59f2e433f1487ec1ac008d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533674Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Commit-Queue: Roman Arora <romanarora@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827277}

chrome/browser/resources/tab_search/tab_search_item.html

@@ -10,6 +10,7 @@
   @media (prefers-color-scheme: dark) {
     :host(:hover, .selected) .button-container cr-icon-button {
       --cr-icon-button-fill-color: var(--google-grey-refresh-300);
+      --cr-icon-button-ripple-opacity: 0.15;
     }
   }
 

ui/webui/resources/cr_elements/mwb_shared_style.html

@@ -22,7 +22,7 @@
     }
 
     ::-webkit-scrollbar-track {
-      background: var(--tab-search-background-color);
+      background: var(--mwb-background-color);
     }
 
     ::-webkit-scrollbar {

ui/webui/resources/cr_elements/mwb_shared_vars.html

@@ -14,8 +14,8 @@
     @media (prefers-color-scheme: dark) {
       html {
         --mwb-background-color: var(--google-grey-900);
-        --mwb-list-item-hover-background-color: var(--google-grey-800);
-        --mwb-list-item-selected-background-color: var(--google-grey-refresh-700);
+        --mwb-list-item-hover-background-color: rgb(55, 56, 58); /* #37383a */
+        --mwb-list-item-selected-background-color: rgb(68, 69, 71); /* #444547 */
       }
     }
   </style>