Can´t add an underline to links in my posts via css - Hello Theme

[Wolf]

I was trying hard to add an underline for my links via css. I just want to have them for the content of the articles.

I don´t know much about css. But I searched and tried several codes in the Theme - Customizer like:
.post-body a:link {
text-decoration: underline;
}

or
.entry-content a: {
text-decoration: underline;
}

But they had no effect at all.

Then I added a css code directly in the Single-Template of my posts. I put the css (a:link {
text-decoration: underline;
}
directly in the post-content-section within the template. I made the same for my link color (where it worked). But with the underline css, every link gets an underline on the whole blogpost (even my main menu). Although i put it just in the "post-content-section".

I would really apreciate some help. I am clueless...