I believe it would be extremely helpful if the background colour of a track could be altered. We ARE able to alter the colour of a wave within a track but it would also be very valuable to be able to control the colour of the track background as well. Please forgive me if I have suggested a feature that is already available and I just didn’t realise it.
Can control all the colours by creating a custom theme, (not simple though).
At https://manual.audacityteam.org/man/themes.html
Themes, Choices, Selecting a Theme, can take me to Custom Theme:
https://manual.audacityteam.org/man/themes.html#custom
which says “The Custom theme to use can be chosen at Edit > Preferences > Interface.”
Not so, this takes me to:
https://manual.audacityteam.org/man/interface_preferences.html
At Interface Preferences, Display, Themes, we are now taken back to Themes.
https://manual.audacityteam.org/man/themes.html
Did I say that right? 
In the Manual it says@
The Custom theme to use can be chosen at Edit > Preferences > Interface.
And yes that “Interface” link properly takes to on to Interface Preferences, where one of the choices for “Theme” is “Custom”. The Custom theme is designed as a template for users to work from.
The next line in the Manual goes on to say
By default the custom theme looks the same as Classic theme - but, if you have the right programming skills and tools, you can use this
template to create your own theme. Instructions for how to do this may be found in the Audacity Wiki.
And the the final two words “Audacity Wiki” are a link to this Wiki page which outlines how to make a custom theme:
And on that Manual page is a hidden ednote (only visisble to editors and only in the alpha Manual) from James which says:
Should those how-to instructions be in the Manual?
James 20Jul17: We (seem to) have agreed that it is ‘for enthusiasts’ and that we direct them to the Wiki, where we can update the instructions and examples as new themes are offered.
HTH
Peter.
There are some user submitted themes in the artwork forum.
Yes, to me the use of the word “Custom” seems to be inconsistent. The important thing is that you are happy with it.
I’m not the developer who provided this - but I suspect his thinking was
a) as delivered there is pseudo “Custom” Theme as a starter point
b) It then forms a hook for a custom theme that a user may produce, a place in Audacity to host and launch that user-custom theme.
Peter.