Clean up `SidebarPanel` CSS

- Perform minor refactor on CSS and remove unnecessary rule

src/styles/sidebar/components/sidebar-panel.scss

 .sidebar-panel {
+  position: relative;
+  background-color: $body-background;
   border: solid 1px $grey-3;
   border-radius: 2px;
-  font-family: $sans-font-family;
   margin-bottom: 0.75em;
-  position: relative;
-  background-color: $body-background;
 
   &__header {
     display: flex;
     flex-direction: row;
     align-items: center;
-    border: 1px none $grey-3;
-    border-bottom-style: solid;
     padding: 1em 0;
     margin: 0 1em;
+    border: 1px none $grey-3;
+    border-bottom-style: solid;
   }
 
   &__title {
@@ -23,6 +22,7 @@
   }
 
   &__close-btn {
+    @include outline-on-keyboard-focus;
     display: flex;
     align-items: center;
     border-radius: 2px;