UX: Fix chat backgrounds (#130)

desktop/desktop.scss

@@ -25,7 +25,6 @@ $sidebar-width: 17em;
   }
 
   body.has-full-page-chat & {
-    grid-template-columns: min-content;
     gap: 0;
   }
 

scss/box-view.scss

@@ -19,20 +19,20 @@
 
 .has-full-page-chat .chat-replying-indicator-container {
   margin-bottom: var(--d-border-radius-large);
-  @include breakpoint(medium) {
+  @include breakpoint(tablet) {
     margin-bottom: 0;
   }
 }
 
 .has-full-page-chat .chat-selection-management {
   margin-bottom: var(--d-border-radius-large);
-  @include breakpoint(medium) {
+  @include breakpoint(tablet) {
     margin-bottom: 0;
   }
 }
 
 .experimental-screen {
-  @include breakpoint(medium) {
+  @include breakpoint(tablet) {
     display: none;
   }
   max-width: unset !important;

scss/header.scss

@@ -6,10 +6,6 @@
   }
 }
 
-.has-full-page-chat .d-header {
-  background-color: transparent;
-}
-
 .user-menu .quick-access-panel,
 .user-notifications-list {
   li {

scss/main.scss

@@ -22,6 +22,7 @@ body {
 
 body.has-sidebar-page.has-full-page-chat #main-outlet-wrapper {
   grid-column-gap: var(--main-grid-gap);
+  background-color: var(--background-color);
 }
 
 body.has-full-page-chat:not(.has-sidebar-page) {