Where can I find the inline-css of my theme header?

That part of the page layout is most often inside of the header.php file in your theme root folder.

But before you edit that file directly, a few advices:

• Do you use a child theme? If not, or you are not sure, than it is not advised to edit that file directly, because when the next update comes out for your theme, your changes will be overwritten by it.
• If you are not familiar with theme development and/or child theme setup, and are looking just for a quick fix, you can add your css through the Customizer, but with the !important clause on the end of each rule. For instance, like this:

.logo-main {
    width: auto!important;
    height: auto!important;
}

This will basically override the inline css for that element with the default values.

• Often the inline css is added by JavaScript (jQuery) using some specific math. If that is the case, don’t be surprised if you don’t find the inline css in your template files.
• As long as the inline styles are not using !important clause, you’ll be fine with your important rules.