Revert change to light theme contrast text color

The change made to whoogle-contrast-text in #873 wasn't the right
decision, since whoogle-contrast-text is meant to contrast with darker
UI elements. whoogle-text already contrasts with the default white
background.

app/static/css/variables.css

@@ -5,7 +5,7 @@
     --whoogle-page-bg: #ffffff;
     --whoogle-element-bg: #4285f4;
     --whoogle-text: #000000;
-    --whoogle-contrast-text: #70757a;
+    --whoogle-contrast-text: #ffffff;
     --whoogle-secondary-text: #70757a;
     --whoogle-result-bg: #ffffff;
     --whoogle-result-title: #1967d2;