RawTherapee GTK+ 3.20 theme improvement

- All buttons now use the same color as the theme's background color.
- ToggleButtons when toggled use a dark color to make it clear that
  they're enabled.
- Fixes #4564

rtdata/themes/RawTherapee-GTK3-20_.css

@@ -222,7 +222,7 @@ button  {
     padding: 0;
     min-height: 5px;
     min-width: 5px;
-    background-image: linear-gradient(#343434, #2E2E2E, #292929);
+    background-image: linear-gradient(to bottom, rgba(100,100,100,.3), rgba(30,30,30,.3));
 }
 
 /* Curve mode buttons and drop-downs */
@@ -247,7 +247,7 @@ radiobutton > radio {
     background-image: linear-gradient(#343434, #2E2E2E, #292929);
 }
 
-button:hover , button.flat:hover, checkbutton:hover > check, radiobutton:hover > radio {
+button.flat:hover, checkbutton:hover > check, radiobutton:hover > radio {
     background-image: linear-gradient(shade(#343434,1.3), shade(#2E2E2E,1.3), shade(#292929,1.3));
 }
 
@@ -255,6 +255,22 @@ button.popupbutton-arrow {
     min-width: 18px;
 }
 
+button:hover  {
+    background-image: linear-gradient(to bottom, rgba(150,150,150,.3), rgba(80,80,80,.3));
+}
+
+button:active  {
+    background-image: linear-gradient(to bottom, rgba(30,30,30,.3), rgba(0,0,0,.3));
+}
+
+button:checked  {
+    background-image: linear-gradient(to bottom, rgba(30,30,30,.8), rgba(0,0,0,.8));
+}
+
+button:checked:hover  {
+    background-image: linear-gradient(to bottom, rgba(60,60,60,1), rgba(30,30,30,1));
+}
+
 /* Save, Cancel, OK ... buttons */
 .dialog-action-area button {
     min-height: 24px;