Avatar size and footer credit removal when using a child theme

I’m trying to change the avatar size to 100px and remove the “Linky plugin powered by Undefined” footer credit. I’m using a child theme, so all the customizations I try to do to Linky won’t work.

CSS I’m using in my child theme’s style.css file:

.linky-page .header__avatar {
    width: 100px;
    height: 100px;
} 
.linky-page .header__avatar img { width: 100%; }
.linky-page footer.footer { display: none; }

The code I’m using in my child theme’s function.php file so the avatar won’t get stretched:

add_action('after_setup_theme', function() {
    add_image_size( 'icon', 100, 100, false );
}, 11);

The problem is that I’m using a child theme since I noticed that the CSS does take effect if I use it in the theme’s customizer within the WP admin.

Any suggestions?

Thanks!