I.e. in general terms: you want to customize your template. How you do that depends first of all on the WordPress theme you have chosen. With most of them you can add individual CSS codes in wp-admin under Appereance > Customizer. Some of them also offer the possibility to specify font sizes for some or all parts of the website. What is possible at this point depends on your theme, which you unfortunately did not reveal. If you have questions about this, you should contact the respective theme developer.
Alternatively you can create a child theme and override the properties of the theme. How to do that is described here: https://developer.www.ads-software.com/themes/advanced-topics/child-themes/
I would recommend not to edit files in the WordPress directory if you don’t know exactly what you are doing. This is especially important against the background of updates. Each update would overwrite individual adjustments. Basically you do something like this only if you have a child theme (see above), develop your own plugin or want to make an adjustment to the basic configuration of WordPress (like the database connection).
If you are concerned about WCAG, you could also choose a WordPress theme that is explicitly designed for its compatibility. Here you can find a selection:
https://www.competethemes.com/blog/wordpress-accessible-themes/
https://colorlib.com/wp/accessible-wordpress-themes/