Change md-checkbox color

in new version added underscore ._md-icon:

md-checkbox.md-checked ._md-icon {
         background-color: white;
}

You need to use theme class as well as md-checked class combination in order to define selector with higher specificity. For example:

md-checkbox .md-icon {
    background: red;
}
md-checkbox.md-default-theme.md-checked .md-icon {
    background: orange;
}

And of course, avoid using !important, this is a sign that something is wrong with your styles.