Improve design of the rich text editor (#9533)

New design for rich text composer

res/css/views/rooms/_MessageComposerButton.pcss

@@ -0,0 +1,68 @@
+/*
+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.
+*/
+
+@define-mixin composerButtonHighLight {
+    background: rgba($accent, 0.25);
+    /* make the icon the accent color too */
+    &::before {
+        background-color: $accent !important;
+    }
+}
+
+@define-mixin composerButton $border-radius,$hover-color {
+    --size: 26px;
+    position: relative;
+    cursor: pointer;
+    height: var(--size);
+    line-height: var(--size);
+    width: auto;
+    padding-left: var(--size);
+    border-radius: $border-radius;
+
+    &::before {
+        content: '';
+        position: absolute;
+        top: 3px;
+        left: 3px;
+        height: 20px;
+        width: 20px;
+        background-color: $icon-button-color;
+        mask-repeat: no-repeat;
+        mask-size: contain;
+        mask-position: center;
+    }
+
+    &::after {
+        content: '';
+        position: absolute;
+        left: 0;
+        top: 0;
+        z-index: 0;
+        width: var(--size);
+        height: var(--size);
+        border-radius: $border-radius;
+    }
+
+    &:hover {
+        &::after {
+            background: rgba($hover-color, 0.1);
+        }
+
+        &::before {
+            background-color: $hover-color;
+        }
+    }
+}