Another way to easily change font sizes is to hold down control and then use the wheel in the middle of your mouse (if it has one and most do).
I have vision issues and a large screen and most websites I use around 110-130% size.
For light on dark text (which I have particular problems with) you can usually click and drag/highlight the area you need to see better. I used to use that a lot back in the early days of the net when a lot of websites had type was that acid green or bright blue on black.