Customizing CSS/JS in a working Typo3 site

This is a Typo3 CMS web site. I need to set the web page so when a user clicks on the icon "Big font" the changes must be: 1) the basic font changes to some font usable by dyslexic people (OmoType or similar) 2) the text,news and fonts got to get bigger. I will give which size to be transformed in (eg. from 1em to 1.6em for news), I will give screenshots. ***Because some dyslexic fonts are smaller then standard ones (eg. Roboto) , if there will be need to set even a bigger font it should be done within this job-project. Before releasing the milestone I will check and agree that the size of fonts are OK. When a user clicks on the icon "Smaller font" the changes made return to normal. That's it. IMPORTANT: Nothing in this working web site should be "broken" after the freelancer makes the changes! Menus, icons and news should work as is now. Please, do the backup of the files you're about to edit (CSS/JS) so if something is not fine that we can revert to normal. This is your duty. If anything is not clear, please ask before do something wrong . Thank you!