How do i change fonts and font size?

I can help you more if you post your site URL.

Just to answer your last, almost every theme have the option to add custom css directly in the options page but if you wanna do heavy customization the right way you wanna do it by making a child theme.

https://codex.www.ads-software.com/Child_Themes

Alright, here is my website: https://reimeiweb.ru/ but it’s pretty empty because i’ve just started it.

I hope you will help me figure out how to fix the fonts ??

Child theme is right way to go.

You can download blank child theme from my site, under heading Toivo Lite version for free.

Here is example how you change fonts via child theme functions.php. Then in child theme style.css something like this.

almost every theme have the option to add custom css directly in the options page

I don’t think it’s even allowed to have this option in w.org themes. And only the Customizer is allowed.