Is your feature request related to a problem? Please describe.

I'm currently developing a browser extension. In my extension, I use Shadow DOM to isolate styles to ensure my extensions render correctly in various web environments. However, I've run into an issue that I hope can be improved.
The problem is that, due to the inability to globally specify the _FontScaleOptions of the euiFontSize method, the font-size of some components uses rem units by default (https://github.com/elastic/eui/blob/main/src/components/tabs/tab.styles.ts). This caused my extension to not render correctly on some webpages with font sizes set.