Font Sizer v1.5.0 is out: set your own base text sizes.
The settings page now has three "Base Text Sizes" inputs: post text, small text, and titles, in pixels. If your theme's font looks too small at Flarum's standard sizes (common with serif fonts), raise these numbers and the whole forum follows, no Custom CSS needed. Changes apply to everyone right away, the font size button keeps working as before and scales on top of what you set, and a live preview shows the effect as you type. Leave the settings at their defaults and nothing changes.
Theme authors can also set the same sizes as CSS variables (--lr-text-base, --lr-text-small, --lr-text-title) from Custom CSS.
Full notes: https://github.com/linkrobins/font-sizer/releases/tag/v1.5.0
Update with composer update linkrobins/font-sizer.