Is your feature request related to a problem? Please describe.

If you create a user.css (and presumably a user.js) in your main template eg cassiopeia then it is used
If you create a child template eg child_cassiopeia then the parent user.css is used
If you create a user.css in your child template then only that child user.css is used

This means that you have to copy everything in the parent user.css to the child user.css

Describe the solution you'd like

a child template should load the parent user.css AND then any child user.css
REMEMBER you may have multiple child templates being used on a site

Additional context - Use Case

add a user.css to cassiopeia to change the size of the h1 tp 50px
add a child green_cassiopeia to change all the color of the h1 to green
add a child red_cassiopeoa to change all the color of the h1 to red

My expected/desired behaviour

Menu item with default cassiopeia tempalte h1=50px
Menu item with green_cassiopeia H1=50px and GREEN
Menu item with red_cassiopeia H1=50px and RED

Currently the only way to achieve that is to copy everything in the main user.css to both the children user.css

This really limits the power of child templates - unless I have missed an undocumented feature