Add borders to critical sidebar components for high-contrast mode

client/src/components/Sidebar/Sidebar.scss

@@ -7,11 +7,12 @@
     color: $color-white;
     width: $sidebar-toggle-size;
     height: $sidebar-toggle-size;
+    box-sizing: border-box;
     background: transparent;
-    border: 0;
     place-items: center;
     padding: 0;
     border-radius: 50%;
+    border: 1px solid transparent;
 
     &:hover {
         background-color: #3a3a3a;
@@ -45,6 +46,10 @@
 
     @include transition(width $menu-transition-duration ease, left $menu-transition-duration ease);
 
+    @media (forced-colors: $media-forced-colours) {
+        border-inline-end: 1px solid transparent;
+    }
+
     &--slim {
         width: $menu-width-slim;
     }

client/src/components/Sidebar/SidebarPanel.scss

@@ -22,6 +22,11 @@
         left: calc(#{$menu-width} - var(--width));
     }
 
+    @media (forced-colors: $media-forced-colours) {
+        border-inline-start: 1px solid transparent;
+        border-inline-end: 1px solid transparent;
+    }
+
     &--visible {
         visibility: visible;
         box-shadow: 2px 0 2px rgba(0, 0, 0, 0.35);