Custom css doesn't work

Hello,

After about 6 hkurs traiing everything found about font size, font color and css I dare to post a question here.

I installed the theme mountain creek and find no way to change font size and color for blog entrys.

I installed plugins for custom css and entered this code:

.single .entry-content,
.single .entry-summary {
font-size: 58px;
font-family: arial;
}

I also tried endless other codings without success.

Somewhen the size of a line in the comment section was changed. So changes take effect if I enter code in this custom css editor.

Here is the link to my site:
https://www.terrakotta-ofen.info

Please please help me somebody. But with specific information I realy spend endless time with no success.

I need to change the size and color of entry text and title in a single blog post and in the overview of the blog entries.

Greetings and thanks in advance

Ralf