Multiple Ui improvements (#6553)

* PLT-6729 - Fixing close button

* PLT-5216 - Updating mattermost theme

* PLT-6679 - Fixing styling for active state

webapp/sass/components/_modal.scss

@@ -213,14 +213,18 @@
                 line-height: 30px;
                 position: absolute;
                 right: 10px;
+                text-shadow: none;
                 width: 30px;
                 z-index: 5;
-                text-shadow: none;
 
                 &:hover {
                     @include alpha-property(background, $black, .1);
                 }
 
+                .sr-only {
+                    display: none;
+                }
+
                 span {
                     font-family: 'Open Sans', sans-serif;
                     line-height: 10px;