fix: replace hardcoded CSS colors with GTK theme variables (v0.7.2)

Greeter used hardcoded colors (#1a1a2e, white, #ff6b6b) instead of
GTK theme variables, breaking theme consistency across the ecosystem.
Now uses @theme_bg_color, @theme_fg_color, @error_color etc. —
matching moonlock and moonset.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moongreet"
-version = "0.7.1"
+version = "0.7.2"
 edition = "2024"
 description = "A greetd greeter for Wayland with GTK4 and Layer Shell"
 license = "MIT"