Custom CSS is not showing changes

[CarlUK]

So I copied the six template and named the folder megasix, and selected this template from within WHMCS admin panel. 

in the custom.css file for that template I've made a few small changes:

/* Change HEADER to some GREEN COLOR FOR TESTING */
.section#header {
    margin: 0;
    padding: 10px 0;
    background-color: #51d2a3 !important;
    border-radius: 5px 5px 0 0;
}

(I've also tried

/* Change HEADER to some GREEN COLOR FOR TESTING */
.header {
    margin: 0;
    padding: 10px 0;
    background-color: #51d2a3 !important;
    border-radius: 5px 5px 0 0;
}

))

Yet no changes I make to this custom.css are showing in any browser whatsoever. 

In Firefox it's reporting that the css file being loaded is  https://megadealhosting.com/billing/templates/megasix/css/all.min.css so it's referencing the right template, but no changes. 

Any ideas?

Carl.