Custom CSS changing all pst body

p, li, .feedback (line 92 in style.css) sets the font size to 90% and font-family to Arial. That’s the reason.

Your Technorati tags are 18px Verdana.

I see, but Custom CSS from Jetpack plugin is supposed to overcome the default css style, isn’t it?
I was used Custom CSS from Jetpack plugin in order to customize my css.
And moreover, why verdana is applied only to technorati and not to all the post’s body?
What I am doing wrong?
Should I edit my css style?
Thanks.

I see, but Custom CSS from Jetpack plugin is supposed to overcome the default css style, isn’t it?

Yes, but you weren’t using specific enough selectors.
https://coding.smashingmagazine.com/2007/07/27/css-specificity-things-you-should-know/

Should I edit my css style?

No.

p is not the same as body. If it’s not specified it will inherit the value from the parent element. But since p is set in style.css this value will be used unless it’s overridden with something that has a higher specificity.

Just replace the CSS you added with your plugin with something like this:

body, p, li, .feedback{
  font-family: verdana, sans-serif;
  font-size: 16px;
}

Many thanks to both your helping posts.
I replaced the CSS with the one you suggested:

body, p, li, .feedback{
font-family: verdana, sans-serif;
font-size: 16px;
}

and that’s it! now display works as I expect.

p.s. I don’t understand what .feedback is there for.

I’d like to take advantage of your kindness once more.
Using che customized:

body, p, li, .feedback{
font-family: verdana, sans-serif;
font-size: 16px;
}

I get the sidebar widgets characters very tiny and it’s difficult to read them.
I see they are:
#menu ul ul li {
font: 70%/1.7em Arial,Verdana,sans-serif;
letter-spacing: 0px;
}
different from the custom CSS
Is there any way to set those fonts bigger at least as the rest of the posts body?
https://lnx.sinapsi.org/wordpress/
Thanks.

Have you tried creating a Custom CSS style like this;

#menu ul ul li {
 font-size: 16px;
}

?

I applied
#menu ul ul li {
font-size: 16px;
}
and now the font of the sidebar widget is bigger, but the line spacing is too big, I don’t understand why, please check yourself:

https://lnx.sinapsi.org/wordpress/

thanks.

It sounds like you need the support of a CSS forum like https://csscreator.com/forum because that query can be resolved by adding your own line height.