CSS images don't show up with bloginfo

Question

I'm new to WP and PHP and I'm trying to convert my 'old' HTML theme into a WP theme. I created my index.php page and linked it to the style.css. I have some images in the css and, from what I've read, I've understood that I have to change my image url so that instead of having:

background-image: url(../assets/Logo.png);

I would have

background-image: url("<?php bloginfo('template_directory'); ?>/assets/Logo.png");

So I made this change on all the urls but when the index loads, even though every CSS element is fine, the images don't appear. However, if I revert them to the html url, the CSS doesn't display at all. And finally, if I have just one bloginfo url in the css and all the others are hmtl, the html ones will load, but the php one won't.

I've changed bloginfo to get_bloginfo (as some suggested) but still no solving. I went through all the parameters related to the path (template_url, stylesheet_directory), but still no change.

I would really appreciate some suggestions or explanations.

Thanks a lot!