Blurry Thumbnails On Mobile

Also my logo on mobile is blurry as well. How can I fix that?

Figured out how to fix Related Posts, I had to edit content-related-post.php and change thumbnails from Medium to Large, but I still have the issue with a blurry mobile logo.

Please send me your site for further checking.
Regards,

You can add the following code to the style.css file:

.wrap-navigation .small-logo {
    background-size: auto auto;
}

#colophon.dark #site-info .small-logo {
    background-size: auto auto;
    width: 130px;
}

Hope this helps !