Style complexity progress bars more heavily

This disables the native progress appearance and uses the green color from our
themes.

res/css/views/elements/_Validation.scss

@@ -14,6 +14,10 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+.mx_Validation {
+    position: relative;
+}
+
 .mx_Validation_details {
     padding-left: 20px;
     margin: 0;