In your theme create a sharedVariable.ftl file and add variable like:
<#assign yourVar = "https://uat.nana.com" />
Then include this ftl file where you need to access this variable like:
<#include "/template/global/include/sharedVariables.ftl" />
Then use the variable as required:
Hope this helps.
P.S. if you are referring some thing in css you usually do it with reference like
background: url("/theme/nana/img/bg_body.jpg") top center repeat-x #fff;
padding: 0 !important;
This should work fine as well and it the the right way of doing it.