Redefine CSS colors in the GTK 3. Improves compatibility with older themes (#604)

data/style/pre-gtk4-variables.scss

+/* Fallback for older CSS themes */
+@define-color cc-bg rgba(46, 46, 46, 0.7);
+
+@define-color noti-border-color rgba(255, 255, 255, 0.15);
+@define-color noti-bg rgba(48, 48, 48, 0.8);
+@define-color noti-bg-opaque rgb(48, 48, 48);
+@define-color noti-bg-darker rgb(38, 38, 38);
+@define-color noti-bg-hover rgb(56, 56, 56);
+@define-color noti-bg-hover-opaque rgb(56, 56, 56);
+@define-color noti-bg-focus rgba(68, 68, 68, 0.6);
+@define-color noti-close-bg rgba(255, 255, 255, 0.1);
+@define-color noti-close-bg-hover rgba(255, 255, 255, 0.15);
+
+@define-color text-color rgb(255, 255, 255);
+@define-color text-color-disabled rgb(150, 150, 150);
+
+@define-color bg-selected rgb(0, 128, 255);

data/style/style.scss

@@ -31,6 +31,9 @@
   --group-collapse-tranistion: opacity 400ms ease-in-out;
 }
 
+/* Fallback for older CSS themes */
+@import "pre-gtk4-variables";
+
 $margin: 8px;
 
 .close-button {