style: replace hardcoded colors with GTK theme variables

Use @theme_bg_color, @theme_fg_color, @error_color and @success_color
instead of hardcoded hex values and 'white'. Makes moonlock respect the
active GTK theme instead of assuming Catppuccin Mocha colors.

Note: moongreet and moonset still use hardcoded colors and should be
updated to match.

resources/style.css

@@ -1,9 +1,9 @@
 /* ABOUTME: GTK4 CSS stylesheet for the Moonlock lockscreen. */
-/* ABOUTME: Dark theme styling matching the Moonarch ecosystem. */
+/* ABOUTME: Uses GTK theme colors for consistency with the active desktop theme. */
 
 /* Main window background */
 window.lockscreen {
-    background-color: #1a1a2e;
+    background-color: @theme_bg_color;
     background-size: cover;
     background-position: center;
     opacity: 0;
@@ -27,14 +27,14 @@ window.lockscreen.visible {
     min-width: 128px;
     min-height: 128px;
     background-color: @theme_selected_bg_color;
-    border: 3px solid alpha(white, 0.3);
+    border: 3px solid alpha(@theme_fg_color, 0.3);
 }
 
 /* Username label */
 .username-label {
     font-size: 24px;
     font-weight: bold;
-    color: white;
+    color: @theme_fg_color;
     margin-top: 12px;
     margin-bottom: 40px;
 }
@@ -46,29 +46,29 @@ window.lockscreen.visible {
 
 /* Error message label */
 .error-label {
-    color: #ff6b6b;
+    color: @error_color;
     font-size: 14px;
 }
 
 /* Fingerprint status indicator */
 .fingerprint-label {
-    color: alpha(white, 0.6);
+    color: alpha(@theme_fg_color, 0.6);
     font-size: 13px;
     margin-top: 8px;
 }
 
 .fingerprint-label.success {
-    color: #51cf66;
+    color: @success_color;
 }
 
 .fingerprint-label.failed {
-    color: #ff6b6b;
+    color: @error_color;
 }
 
 /* Confirmation prompt */
 .confirm-label {
     font-size: 16px;
-    color: white;
+    color: @theme_fg_color;
     margin-bottom: 4px;
 }
 
@@ -103,12 +103,12 @@ window.lockscreen.visible {
     min-height: 48px;
     padding: 0px;
     border-radius: 24px;
-    background-color: alpha(white, 0.1);
-    color: white;
+    background-color: alpha(@theme_fg_color, 0.1);
+    color: @theme_fg_color;
     border: none;
     margin: 4px;
 }
 
 .power-button:hover {
-    background-color: alpha(white, 0.25);
+    background-color: alpha(@theme_fg_color, 0.25);
 }