chore: replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

app/utils/to-electron-background-color.ts

@@ -13,5 +13,5 @@ export default (bgColor: string) => {
 
   // http://stackoverflow.com/a/11019879/1202488
   const alphaHex = Math.round(color.alpha() * 255).toString(16);
-  return `#${alphaHex}${color.hex().toString().substr(1)}`;
+  return `#${alphaHex}${color.hex().toString().slice(1)}`;
 };