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I'm really counting on your help in this one. I searched a lot and found no solution. I want to have a custom icon for my plugin in admin menu, and I want it to integrate well with the color scheme.

I looked here https://codex.wordpress.org/Function_Reference/add_menu_page Under $icon_url

(WP 3.8+) If 'data:image/svg+xml;base64...', the specified SVG data image is used as a CSS background

However, if I put a URL to an SVG icon there, all I get is an img with SVG in its src attribute, so it doesn't integrate at all with the color scheme. It's supposed to be a CSS background.

Also, I don't understand this data:image/svg+xml;base64... What does it mean exactly?

I tried embedding inline SVG in the $icon_url but obviously, it won't work, but I just had to try it.

Dashicons are not an option, there's no icon there suitable for my project.

Answers

I got the solution by analyzing Woocommerce. If no url is supplied to the add_menu_page function, WordPress uses the default dashicon. So it's just a matter of overriding the default style. Like add this to admin styles:

#adminmenu #toplevel_page_yourpageid .menu-icon-generic div.wp-menu-image:before {
    font-family: your_font !important;
    content: '\eiconid';
    font-size: 1.3em!important;
}

I converted svg to font on Icomoon.