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">