GTK 3.20, 3.22: add some gnome-tweak-tool styling

common/gtk-3.0/3.20/gtk-dark.css

@@ -3393,6 +3393,10 @@ button.documents-favorite:active:hover {
   opacity: 0.0;
   transition: opacity 0.2s ease-out; }
 
+.tweak-categories,
+.tweak-category:not(:selected):not(:hover) {
+  background-image: linear-gradient(to bottom, #404552, #404552); }
+
 .tr-workarea undershoot,
 .tr-workarea overshoot {
   border-color: transparent; }

common/gtk-3.0/3.20/gtk-darker.css

@@ -3392,6 +3392,10 @@ button.documents-favorite:active:hover {
   opacity: 0.0;
   transition: opacity 0.2s ease-out; }
 
+.tweak-categories,
+.tweak-category:not(:selected):not(:hover) {
+  background-image: linear-gradient(to bottom, #ffffff, #ffffff); }
+
 .tr-workarea undershoot,
 .tr-workarea overshoot {
   border-color: transparent; }

common/gtk-3.0/3.20/gtk-solid-dark.css

@@ -3393,6 +3393,10 @@ button.documents-favorite:active:hover {
   opacity: 0.0;
   transition: opacity 0.2s ease-out; }
 
+.tweak-categories,
+.tweak-category:not(:selected):not(:hover) {
+  background-image: linear-gradient(to bottom, #404552, #404552); }
+
 .tr-workarea undershoot,
 .tr-workarea overshoot {
   border-color: transparent; }

common/gtk-3.0/3.20/gtk-solid-darker.css

@@ -3392,6 +3392,10 @@ button.documents-favorite:active:hover {
   opacity: 0.0;
   transition: opacity 0.2s ease-out; }
 
+.tweak-categories,
+.tweak-category:not(:selected):not(:hover) {
+  background-image: linear-gradient(to bottom, #ffffff, #ffffff); }
+
 .tr-workarea undershoot,
 .tr-workarea overshoot {
   border-color: transparent; }

common/gtk-3.0/3.20/gtk-solid.css

@@ -3396,6 +3396,10 @@ button.documents-favorite:active:hover {
   opacity: 0.0;
   transition: opacity 0.2s ease-out; }
 
+.tweak-categories,
+.tweak-category:not(:selected):not(:hover) {
+  background-image: linear-gradient(to bottom, #ffffff, #ffffff); }
+
 .tr-workarea undershoot,
 .tr-workarea overshoot {
   border-color: transparent; }

common/gtk-3.0/3.20/gtk.css

@@ -3396,6 +3396,10 @@ button.documents-favorite:active:hover {
   opacity: 0.0;
   transition: opacity 0.2s ease-out; }
 
+.tweak-categories,
+.tweak-category:not(:selected):not(:hover) {
+  background-image: linear-gradient(to bottom, #ffffff, #ffffff); }
+
 .tr-workarea undershoot,
 .tr-workarea overshoot {
   border-color: transparent; }

common/gtk-3.0/3.20/sass/_applications.scss

@@ -501,6 +501,14 @@ button.documents-favorite:active:hover {
     transition: opacity 0.2s ease-out;
 }
 
+//
+// Gnome Tweak Tool
+//
+.tweak-categories,
+.tweak-category:not(:selected):not(:hover) {
+  background-image: _solid($base_color);
+}
+
 //
 // Transmission
 //