Customizing ProductCart - Global Increase Font Sizes in v5.2.10

Mobile Usability issues detected on https://www.endoscopy.com/" rel="nofollow - https://www.endoscopy.com/
To the owner of https://www.endoscopy.com/:" rel="nofollow - https://www.endoscopy.com/:
Search Console has identified that your site is affected by 1 Mobile Usability issues:

Top Issues

The following issues were found on your site:

Text too small to read

We recommend that you fix these issues when possible to enable the best experience and coverage in Google Search.

I use "Bootstrap Blue v3" for our site so I don't have to have separate "Desktop" and "Mobile" sites to maintain. Fonts are supposed to be controlled with "theme.css", however I found that there are many different font sizes throughout the CSS code depending on what part of ProductCart it handles.

Is there a CSS code line that I can edit, or add to "theme.css" to GLOBALLY (site-wide) bump up the display of all fonts/text (all Header <hx> or <p> tags) on the site to make everything appear a little bit larger than the current font size? Perhaps designed to bump the font size ONLY when the screen resolution reaches 800 pixels or smaller (to target Mobile displays only)?