Background Color Change Switch Not Working

Question

I'm trying to make a dark/light theme. When I click to flip into light, the function runs. but darken(); seems to be ignored. Am I missing something? no error messages.

<label class="switch">
    <input type="checkbox">
<span class="slider round" onclick="contrast();"></span>

var m = 1

function lightup() {
    document.getElementById('header-wrap').style.backgroundColor = "white"
}

function darken() {
    document.getElementById('header-wrap').style.backgroundColor = 'black'
}

function contrast() {
    if (m = 1) {
        m = 2
        lightup();
    }
    else if (m = 2) {
        m = 1
        darken();
    }
}

contrast(); and lightup(); both fire as intended. darken(); does not.