1400.modifyVars.less

@theme: default;
@import "@{theme}.less";

blue.less

#rectangle {
    background: blue;
} 

config.less

@theme: default;

default.less

#rectangle {
    background: gray;
}

framework.less

@import "config.less";
@import "@{theme}.less";

green.less

#rectangle {
    background: green;
}

red.less

#rectangle {
    background: red;
}