First attemp

gtk-3.0/gtk-dark.css

@@ -4045,6 +4045,8 @@ placessidebar.sidebar {
     padding-left: 14px;
     padding-right: 12px;
     color: #9ecfa2; }
+    placessidebar.sidebar row.sidebar-row.sidebar-row .sidebar-icon:first-child {
+      background: #171e27; }
   placessidebar.sidebar row.sidebar-row:hover {
     transition: all 300ms cubic-bezier(0.25, 0.46, 0.45, 0.94);
     color: rgba(0, 0, 0, 0.76);
@@ -4067,9 +4069,13 @@ placessidebar.sidebar {
       color: rgba(0, 0, 0, 0.54);
       background-color: transparent;
       background-image: linear-gradient(to right, #c50ed2 40px, rgba(65, 67, 75, 0) 36px, rgba(65, 67, 75, 0) 97%); }
+      placessidebar.sidebar row.sidebar-row:selected:backdrop .sidebar-icon:first-child {
+        background: #c50ed2; }
     placessidebar.sidebar row.sidebar-row:selected .sidebar-icon {
       -gtk-icon-shadow: 0 1px rgba(255, 255, 255, 0.8);
       color: inherit; }
+      placessidebar.sidebar row.sidebar-row:selected .sidebar-icon:first-child {
+        background: #c50ed2; }
 
 placessidebar list {
   background-color: transparent; }

gtk-3.0/gtk.css

@@ -4045,6 +4045,8 @@ placessidebar.sidebar {
     padding-left: 14px;
     padding-right: 12px;
     color: #9ecfa2; }
+    placessidebar.sidebar row.sidebar-row.sidebar-row .sidebar-icon:first-child {
+      background: #171e27; }
   placessidebar.sidebar row.sidebar-row:hover {
     transition: all 300ms cubic-bezier(0.25, 0.46, 0.45, 0.94);
     color: rgba(0, 0, 0, 0.76);
@@ -4067,9 +4069,13 @@ placessidebar.sidebar {
       color: rgba(0, 0, 0, 0.54);
       background-color: transparent;
       background-image: linear-gradient(to right, #c50ed2 40px, rgba(65, 67, 75, 0) 36px, rgba(65, 67, 75, 0) 97%); }
+      placessidebar.sidebar row.sidebar-row:selected:backdrop .sidebar-icon:first-child {
+        background: #c50ed2; }
     placessidebar.sidebar row.sidebar-row:selected .sidebar-icon {
       -gtk-icon-shadow: 0 1px rgba(255, 255, 255, 0.8);
       color: inherit; }
+      placessidebar.sidebar row.sidebar-row:selected .sidebar-icon:first-child {
+        background: #c50ed2; }
 
 placessidebar list {
   background-color: transparent; }

gtk-3.0/widgets/_sidebar.scss

@@ -135,7 +135,10 @@ placessidebar{
 						margin-right: 12px;
 						padding-left: 14px;
 						padding-right: 12px;
-						color: #9ecfa2;
+            color: #9ecfa2;
+            &:first-child {
+              background: #171e27;
+            }
 					}
 				}
 				&:hover{
@@ -169,11 +172,19 @@ placessidebar{
             background-color: transparent;
             background-image:linear-gradient(to right, $selected_bg_color 40px,
                                   rgba(65,67,75,0) 36px, rgba(65,67,75,0)97%);
+            .sidebar-icon {
+              &:first-child {
+                background: $selected_bg_color;
+              }
+            }
           }
 
 					.sidebar-icon {
 						-gtk-icon-shadow: 0 1px transparentize(#fff, 0.2);
-						color: inherit; 
+            color: inherit; 
+            &:first-child {
+              background: $selected_bg_color;
+            }
 					}
 				}