Improve tab styles on the authentication forms

Previously the active tab would be red, which could be confused with our
red anchor text and make the other tab look disabled. The new design
makes all text gray but adds an "active" border below the button.

h/static/styles/common.scss

@@ -416,7 +416,7 @@ blockquote {
   background-color: $body-background;
   border: 1px none $gray-lighter;
   border-bottom-style: solid;
-  padding: 1em;
+  padding: 0.9em 1em 1.1em;
 
   & > li {
     display: inline-block;
@@ -427,10 +427,21 @@ blockquote {
       font-weight: bold;
       color: $gray-dark;
       cursor: pointer;
+      border-bottom: 3px solid $gray-lighter;
+      padding-left: 2 / 13 * 1em;
+      padding-right: 2 / 13 * 1em;
+      padding-bottom: 3 / 13 * 1em;
     }
 
-    &.active a, &:active a {
-      color: $hypothered;
+    &:active a {
+      position: relative;
+      top: 1 / 13 * 1em;
+      padding-bottom: 2 / 13 * 1em;
+    }
+
+    &:active a,
+    &.active a {
+      border-color: $hypothered-desat;
     }
 
     &:before {
@@ -439,8 +450,7 @@ blockquote {
     }
 
     &:first-child:before {
-      content: "";
-      margin: 0;
+      content: none;
     }
   }
 }