Conform to WCAG 2 AA contrast minimums for Google OAuth.

Also apply variables for FB/Linkedin OAuth2 rather than
leave color hashes hanging around.

lms/static/sass/multicourse/_account.scss

@@ -534,7 +534,7 @@
       }
 
       &.button-oa2-google-oauth2:hover, &.button-oa2-google-oauth2:focus {
-        background-color: #dd4b39;
+        background-color: $google-red;
         border: 1px solid #A5382B;
       }
 
@@ -543,7 +543,7 @@
       }
 
       &.button-oa2-facebook:hover, &.button-oa2-facebook:focus {
-        background-color: #3b5998;
+        background-color: $facebook-blue;
         border: 1px solid #263A62;
       }
 
@@ -552,7 +552,7 @@
       }
 
       &.button-oa2-linkedin-oauth2:hover , &.button-oa2-linkedin-oauth2:focus {
-        background-color: #0077b5;
+        background-color: $linkedin-blue;
         border: 1px solid #06527D;
       }
 

lms/static/sass/partials/base/_variables.scss

@@ -200,7 +200,7 @@ $ui-notification-height: ($baseline*10);
 $twitter-blue: #55ACEE;
 $facebook-blue: #3B5998;
 $linkedin-blue: #0077B5;
-$google-red: #DD4B39;
+$google-red: #D73924;
 $microsoft-blue: #00BCF2;
 
 // shadows