JS Error when clicking on Preferences > Custom CSS

[SiL3NC3]

I want to increase the font-size for the entire app - i use the portable version. So the idea was to create custom css with font-size definition.

But after clicking Preferences > Custom CSS, an error comes up:

Uncaught Exception:
TypeError: shell.openItem is not a function 
at click (resources/app.asar/menu.js:42:19)
at MenuItem.click (electron/js2c/browser_init.js:73:1742)

Any help appreciated.

Alternatively allow Ctrl + MouseWheel to resize the app.