Add env var for background-85

Signed-off-by: Šimon Brandner <simon.bra.ag@gmail.com>

.env.example

@@ -27,3 +27,4 @@
 # VITE_THEME_QUINARY_CONTENT=#394049
 # VITE_THEME_SYSTEM=#21262c
 # VITE_THEME_BACKGROUND=#15191e
+# VITE_THEME_BACKGROUND_85=#15191ed9

src/main.tsx

@@ -84,6 +84,10 @@ if (import.meta.env.VITE_CUSTOM_THEME) {
     "--background",
     import.meta.env.VITE_THEME_BACKGROUND as string
   );
+  style.setProperty(
+    "--background-85",
+    import.meta.env.VITE_THEME_BACKGROUND_85 as string
+  );
 }
 
 const history = createBrowserHistory();