customise the font size in the dashboard

You can only define your custom CSS which would override default WordPress fonts. This is not supported by AGCA and you would need to define this by yourself. You can add those customizations in AGCA -> Advanced tab -> custom CSS

OK great!
What should I write in CSS to set the font size?

waht the name of the class?

That might not be an easy task to do. You can try with this simple change first. Everything else you would need to define by yourself.

body{ font-size: 18px !important}

Thanks,
Argonius

OK

what do you think about this code?

.wp-admin {font-size: 120% !important; }

#adminmenu .wp-submenu a {font-size: 16px; line-height: 28px;}

#adminmenu a.menu-top, #adminmenu .wp-submenu-head {font-size: 18px;}

#adminmenu .wp-not-current-submenu li a {padding-top: 11px;}
#adminmenu a.menu-top, #adminmenu .wp-submenu-head {line-height: 28px;}

In fact, a few minutes ago I received an error message from AG plugin and I ignored it, and then the custom colors stopped working
What could be the reason??

It looks good. It’s good as long as you and your clients are satisfied with it.

please check browser’s console for errors