Self-contained CSS for context menus. For the updated/completed theme please use: https://github.com/Guerra24/Firefox-UWP-Style

win10-context-menus.css

menupopup {
	-moz-appearance: none !important;
	padding: 8px 2px 8px 2px !important;
	background-color: #2B2B2B !important;
	border: 1px #A0A0A0 solid !important;
	min-width: 280px !important;
}
menuseparator {
	-moz-appearance: none !important;
	border: none !important;
	padding: .5px 0px .5px 0px !important;
	margin: 4px 8px 4px 8px !important;
	background-color: #808080 !important;
}
menuitem, menu {
	-moz-appearance: none !important;
	color: white !important;
	padding: 7.5px 0px 7.5px 0px !important;
}
menuitem[disabled="true"], menuitem[disabled="true"] > .menu-text {
	color: gray !important;
}
menuitem:hover, menu:hover, menu[open="true"], menuitem[_moz-menuactive="true"]	 {
	-moz-appearance: none !important;
	padding: 7.5px 0px 7.5px 0px !important;
	background-color: #414141 !important;
}
menugroup {
	padding: 0px !important;
	background-color: transparent !important;
}
menuitem > .menu-text, menuitem > .menu-iconic-text {
	color: white !important;
}
menuitem[type="checkbox"] {
	padding-left: 8px !important;
	padding-top: 6.5px !important;
	padding-bottom: 6.5px !important;
}
menuitem[type="radio"] {
	padding-left: 8px !important;
	padding-top: 6.5px !important;
	padding-bottom: 6.5px !important;
}
menuitem[type="checkbox"] > .menu-iconic-text, menuitem[type="radio"] > .menu-iconic-text {
	padding-left: 5px !important;
}
menuitem[checked="true"][type="checkbox"] > .menu-iconic-left {
	padding: 7px 0px 0px 0px !important;
	border: solid white;
	border-width: 0 0 2px 2px;
	transform: rotate(-45deg) scale(.75);
	margin-bottom: 4px !important;
}
menuitem[checked="true"][type="radio"] > .menu-iconic-left {
	padding: 1px !important;
	border: solid white;
	height: 17px !important;
	border-radius: 100%;
	background: white;
	transform: scale(.5); 
}
menucaption {
	background-color: transparent !important;
	color: white !important;
}
#ContentSelectDropdown > menupopup {
	background-image: none !important;
}
#ContentSelectDropdown {
	padding: 0px !important;
}
menulist {
	-moz-appearance: none !important;
	padding: 4px 4px 4px 4px !important;
	border: 2px solid #666666 !important;
	background-color: transparent !important;
}
menulist:hover {
	padding: 4px 4px 4px 4px !important;
	border: 2px solid #999999 !important;
	background-color: transparent !important;
}
menulist[open="true"]  {
	border: 2px solid #666666 !important;
	background-color: #333333 !important;
}
#context-back > .menu-iconic-left, #context-forward > .menu-iconic-left, #context-reload > .menu-iconic-left, #context-stop > .menu-iconic-left, #context-bookmarkpage > .menu-iconic-left {
	padding: 0px !important;
}
.menu-iconic-left {
	-moz-appearance: none !important;
	padding: 0px 5px 0px 9px !important;
}
.menu-right {
	-moz-appearance: none !important;
	padding: 7.5px !important;
	margin-right: 6px !important;
	color: white !important;
	border: solid white;
	border-width: 0 2px 2px 0px;
	transform: rotate(-45deg) scale(.55);
}
tooltip {
	-moz-appearance: none !important;
	color: white !important;
	background-color: #2B2B2B !important;
	border: 1px solid #767676 !important;
	padding: 6.5px !important;
}

i'm proud of you...well done.
thanks for sharing!

For anyone having problems with the bookmarks menu (e.g. extremely large rows), try adding block this to the bottom:

#bookmarks-menu-button .menu-right {
	-moz-appearance: unset !important;
	padding: unset !important;
	margin-right: unset !important;
	border-width: unset !important;
	border: unset !important;
	border-width: unset !important;
	transform: none !important;
}