Improve composer visiblity (#8578)

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

res/css/views/buttons/_Cancel.scss

@@ -0,0 +1,32 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.mx_CancelButton {
+    width: var(--size);
+    height: var(--size);
+
+    box-sizing: border-box;
+    padding: calc(var(--size) / 4);
+    border-radius: 50%;
+
+    line-height: 0;
+    background: $quinary-content;
+
+    svg {
+        width: calc(var(--size) / 2);
+        color: $secondary-content;
+    }
+}