How to use Custom CSS to add into on all pages?

Thanks!
1. header.php
Thanks for pointing me the right diretion!
I actually figured out how to add a <link> to header.php last night, so I ma happy that I did the right thing :)))

2. Remove favicon from settings
I removed the favicon.ico from the Theme Setting -> General page.

But this link still remains in the Basel theme code, I cannot see how to remove it because it is not in header.php. So when I remove favicon.ico from the Theme setting, it overrides and tries to provide Basel default favicon:
<link rel=”shortcut icon” href=”https://www.independent.wine/wp-content/themes/basel/images/icons/favicon.png”&gt;

You can see it in the view source of the page: view-source:https://www.independent.wine/

<!–[if lt IE 9]>

<![endif]–>
<link rel=’https://api.w.org/&#8217; href=’https://www.independent.wine/wp-json/&#8217; />
<style type=”text/css”>
#wp-admin-bar-assetcleanup-parent span.dashicons {
width: 15px;
height: 15px;
font-family: ‘Dashicons’, Arial, “Times New Roman”, “Bitstream Charter”, Times, serif !important;
}

#wp-admin-bar-assetcleanup-parent > a:first-child strong {
font-weight: bolder;
color: #76f203;
}

#wp-admin-bar-assetcleanup-parent > a:first-child:hover {
color: #00b9eb;
}

#wp-admin-bar-assetcleanup-parent > a:first-child:hover strong {
color: #00b9eb;
}

#wp-admin-bar-assetcleanup-test-mode-info {
margin-top: 5px !important;
margin-bottom: -8px !important;
padding-top: 3px !important;
border-top: 1px solid #ffffff52;
}

/* Add some spacing below the last text */
#wp-admin-bar-assetcleanup-test-mode-info-2 {
padding-bottom: 3px !important;
}
</style>
<meta name=”referrer” content=”always”/><meta name=”theme-color” content=””> <link rel=”shortcut icon” href=”https://www.independent.wine/wp-content/themes/basel/images/icons/favicon.png”&gt;
<link rel=”apple-touch-icon-precomposed” sizes=”152×152″ href=”https://www.independent.wine/wp-content/uploads/2019/08/android-chrome-192×192-1.png”&gt;
<style>

The snippet does not save:

The snippet has been deactivated due to an error on line 2:

Cannot redeclare function basel_favicon.

Artem, hi,
It worked. I posted the code directly into functions.php instead of a Snippet, and it saved fine. Thanks for your help!

Yesterday I downloaded Google fonts I use and posted them as Custom fonts. This removed a few http requests. Today, I removed the favicon.png request and replaced it with binary.

Now, I am at 1.0 – 1.2 seconds load time (GTMEtrix) and at 50/95 with Google speed checker
https://ibb.co/NZSRcwN

I work with a combo of Asset Optimiser + WP Rocket + SG Optimiser. As you see, my YSlow is 85% because I do not use a CDN (PageSpeed is 98%). I assume that by going for CDN I may gain a bit further.

If you have any other optimisation suggestions please share 🙂

Have a nice weekend, and thanks for the fast theme.