muk_web/muk_web_theme/static/src/webclient/appsmenu/appsmenu.xml

<?xml version="1.0" encoding="UTF-8" ?>

<templates xml:space="preserve">

	<t 
		t-name="muk_web_theme.AppsMenu" 
		t-inherit="web.Dropdown" 
		t-inherit-mode="primary" 
		owl="1"
	>
		<xpath expr="//div[@t-ref='menuRef']" position="attributes">
			<attribute name="t-attf-style">
	        	background-image: url("{{ backgroundImageUrl }}");
		    </attribute>
	    </xpath>
	</t>
	
</templates>