themes: Define symbolic icon colors for System theme

Symbolic layer tree icons are often invisible
when not selected on System themes in
light mode. This patch adds a CSS rule
to ensure the layer tree icons are visible
on light backgrounds.

themes/System/gimp-light.css

@@ -0,0 +1,13 @@
+/* GIMP theme CSS file */
+/* Additional rules for light variant System themes */
+
+@import url("gimp.css");
+
+/* Make sure symbolic layer tree icons are visible */
+treeview {
+  color: black;
+}
+
+treeview header {
+  color: rgb (196, 197, 197);
+}

themes/System/meson.build

@@ -0,0 +1,6 @@
+files = [
+  'gimp-light.css'
+]
+
+install_data(files,
+             install_dir: gimpdatadir / 'themes' / 'System')

themes/meson.build

@@ -1,4 +1,5 @@
 subdir('Default')
+subdir('System')
 
 themes = [
   'System',