calc
Since UXP v3.0
You can use calc to evaluate arbitrary expressions in the context of a CSS style rule. 
See: https://developer.mozilla.org/en/docs/Web/CSS/calc
Example
Copied to your clipboard.someElement {width: calc(100vh - 50px);}
Quirks and Exceptions
- calcworks for length values, not colors. However, you can use- calcfor the numeric portions of an- rgb()color.