Changing Colour of 'To' and 'Subject' windows

Could someone help - in the compose window the ‘to’ and ‘subject’ sections are both coloured white and when I attempt to enter anything in these sections the text also appears as white and is obviously unreadable.
How do I change this - presumably using the theme editor but have no idea where to find the relevant section.

Theme Editor
Change TextBoxBackground color
or Change TextBoxforeground color to change color of text

You can click apply to see what it will look like

You’ll have to save then import to change the theme

Thank you for your response - that works.
Much appreciared