Pico with bootstrap's grid system would be a dream.
Bootstrap grid system is super complete, and probably better coded that anything I can make. Its also available as a standalone module, so the size is pretty small. I just wanted it integrated to PicoCSS because I'm lazy.
@media (min-width: 800px) {
#page {
grid-template-areas:
"header header"
"menu content"
"footer footer";
}
}
@media (max-width: 800px) {
#page {
grid-template-areas:
"header menu"
"content content"
"footer footer";
}
}