waybar: fix GTK3 CSS variables, pin timezone, adjust screenshot keybind

- theme/colors.css: replace CSS custom properties (--var) with GTK3
  @define-color — GTK3 CSS parser rejects -- syntax, crashing waybar
- waybar/style.css, wofi/style.css, gtklock/style.css: update var(--x)
  references to @x to match
- waybar/config.jsonc: pin clock timezone to America/Vancouver
- niri/config.kdl: change Print to Mod+Print for screenshot keybind

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

waybar/style.css

@@ -6,17 +6,17 @@
 }
 
 window#waybar {
-  background: var(--tn-bg-a90);
-  color: var(--tn-fg);
+  background: @tn-bg-a90;
+  color: @tn-fg;
 }
 
 #workspaces button {
   padding: 0 8px;
-  color: var(--tn-fg-muted);
+  color: @tn-fg-muted;
 }
 
 #workspaces button.focused {
-  color: var(--tn-blue);
+  color: @tn-blue;
 }
 
 #clock, #battery, #network, #pulseaudio,
@@ -26,33 +26,33 @@ window#waybar {
 }
 
 #custom-power-profile.performance {
-  color: var(--tn-yellow);
+  color: @tn-yellow;
 }
 
 #custom-power-profile.power-saver {
-  color: var(--tn-green);
+  color: @tn-green;
 }
 
 #battery.warning {
-  color: var(--tn-yellow);
+  color: @tn-yellow;
 }
 
 #battery.critical {
-  color: var(--tn-red);
+  color: @tn-red;
 }
 
 #cpu.warning {
-  color: var(--tn-yellow);
+  color: @tn-yellow;
 }
 
 #cpu.critical {
-  color: var(--tn-red);
+  color: @tn-red;
 }
 
 #temperature.warm {
-  color: var(--tn-yellow);
+  color: @tn-yellow;
 }
 
 #temperature.critical {
-  color: var(--tn-red);
+  color: @tn-red;
 }