Transfer between Macs

Hi, is there anything special I need to do to copy a theme from one Mac to another?
I have tried copying the theme folder over and installing it and have also tried copying it from the location Mailspring stores them. Using both methods, I lose the colour of the toolbar at the top!

I set this in the ui-variables file

@toolbar-background-color: #16085b;
@panel-background-color: #16085b;

On the Mac I copied it to, the toolbar is greyish.

Untitled (16)

Does anybody know why this is happening?