For more control than just an Fn key you might look at a macro app like Keyboard Maestro . It can do anything you can do with just a shot keystroke combination. like changing the font size and engaging a CSS file in Safari:

I use it to launch apps, enter predetermined text in documents and replies like this one, change system preferences, etc. I believe Keyboard Maestro is the more powerful of the macro apps but there are some in the App Store that might meed your needs. Go there and search for macro.
The following text, which has nothing to do with this topic, was added by typing this key combination: x2cov
Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?
Just and example of what fan be done.
