Nice & simple

Works great, thanks a lot.

One thing though, somehow within my theme the alignment to the left didn’t work out. margin-left: 0 !important; wasn’t enough, it seems. I fixed it by adding padding-left: 0! important; to .collapsed-archives > ul { }

Also made a modification to add a fade/transition effect to the dropdown.
For those interested it’s just some addition the last few lines of code:

/* Toggled content */
.collapsed-archives input ~ ul {
	display: inline-block;
    opacity: 0;
    transition: opacity 0s linear 0s;
    -webkit-transition: opacity 0s linear 0s;
}
.collapsed-archives input:checked ~ ul {
    opacity: 1;
    display: block;
    transition: opacity 0.5s linear 0s;
    -webkit-transition: opacity 0.5s linear 0s;
}