Use variables for the rem values.

It's become obvious that these random floating points everywhere
are unwieldy. Now they're all in one place with some fairly logical
variable names which will help out in design->implementation phase.

res/css/views/elements/_Tooltip.scss

@@ -58,8 +58,8 @@ limitations under the License.
     z-index: 4000; // Higher than dialogs so tooltips can be used in dialogs
     padding: 10px;
     pointer-events: none;
-    line-height: 0.933rem;
-    font-size: 0.8rem;
+    line-height: $font-14px;
+    font-size: $font-12px;
     font-weight: 600;
     color: $primary-fg-color;
     max-width: 200px;
@@ -82,7 +82,7 @@ limitations under the License.
     text-align: center;
     border: none;
     border-radius: 3px;
-    font-size: 0.933rem;
+    font-size: $font-14px;
     line-height: 1.2;
     padding: 6px 8px;