don't set a default avatar size in the css, makes it easier to change it

src/platform/web/ui/css/avatar.css

@@ -16,7 +16,6 @@ limitations under the License.
 */
 
 .avatar {
-    --avatar-size: 32px;
     width: var(--avatar-size);
     height: var(--avatar-size);
     overflow: hidden;

src/platform/web/ui/css/themes/element/theme.css

@@ -250,6 +250,7 @@ a.button-action {
     /* make scrollbar hit right edge of parent */
     padding-right: 8px;
     margin-right: -8px;
+    --avatar-size: 32px;
 }
 
 .RoomList > li {
@@ -434,6 +435,10 @@ a {
     background-position-x: 10px;
 }
 
+.RoomHeader {
+    --avatar-size: 32px;
+}
+
 .RoomHeader .topic {
     font-size: 14rem;
 }