I'm trying to style an Avalonia Expander control but I'm unable to fully figure out how to style the header. My main difficulty is styling the chevron button. I'm not sure quite what you call it, but I've marked it on this image to show what I mean on an unstyled control.

The code I have for my own control looks like this (ignore the wild colours, that was so I could easily distinguish different parts of the control easily while experimenting):

    
        
        
    

    
        
            
            
            
            
            
            
            
            
            
            
        
    

    
        
            This is the header
        
        
            This is the first label of the expander
            This is the second label of the expander
            This is the third label of the expander
            This is the fourth label of the expander
        
    

So, how do I style this square "chevron button"? Everything I've tried or found has not worked or I don't understand how to extrapolate the information given to my situation (a lot of answers just point at the source code for the control, which I can't map between that and what I need to do to style it in my code).

I want to change the colour of the button and the chevron when displaying normally, when hovered over and when expanded.

Also, if there is anything about the way I've used the existing styles that could be improved on I'm happy to take advice on that too.