With @layer this has become simpler:

    /* define layer order. Not necessary here */
    @layer pico, nils;

    /* import framework into layer */
    @import url("pico.css") layer(pico);

    /* define own styles in own layer, they have precedence. CSS vars are still global. */
    @layer nils {
        p:first-of-type {
            color: green;
        }
    }