foo.less

.foo {
    background: @red;
}

main.less

@import-once "variables";
@import-once "foo";

style.less.css

/* this is what is embedded as /assets/styles/style.less-d7b9d09e31714142e13736e06bda6e84.css" */
.foo {
  background: #ff0000;
}
@import-once "variables";
@import-once "foo";

variables.less

@red: red;