MudSelectExtended: style or class not working for MudSelectItemGroupExtended

[SilentRage47]

Can't change the style of the header group in MudSelectedExtended.

Neither of this works:

<MudSelectItemGroupExtended T="User" Class="header" Text="item.Name" Sticky="false" Nested="false" InitiallyExpanded="true">
....
<style>
	.header
	{
		font-weight: 900 !important;
		background-color: red !important;
	}
</style>

<MudSelectItemGroupExtended T="User" Style="font-weight:700" Text="@item.Department.Description" Sticky="false" Nested="false" InitiallyExpanded="true">