[gimp] themes: prefer symbolic icons when available.

[Jehan <jehanp src gnome org>]

commit cba95fb8a5bcdbab36c946827b934cc761b702c3
Author: Jehan <jehan girinstud io>
Date:   Mon May 28 20:48:55 2018 +0200

    themes: prefer symbolic icons when available.
    
    This is a first step to make our 2 symbolic themes into one and properly
    "announce" them as symbolic through icon naming (which will allow
    recoloring according to style colors).
    With this CSS style, GTK+ widgets will search for symbolic icon variants
    when using the generic name (with "*-symbolic" suffix).

 themes/System/gimp.css | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/themes/System/gimp.css b/themes/System/gimp.css
index 7c5b4d6673..d296ffa400 100644
--- a/themes/System/gimp.css
+++ b/themes/System/gimp.css
@@ -1,6 +1,7 @@
 /* GIMP theme css file */
 
 * {
+    -gtk-icon-style:                   symbolic;
     -GimpDockWindow-default-height:    300;
     -GimpMenuDock-minimal-width:       200;
     -GimpDockWindow-menu-preview-size: button;