It's primary and not Primary. You can always check on the inspect elements with the chrome developer tools the classes of the elements, that way is easier to change something

.btn-primary, .login-form.btn-primary {
    border-color: #222;
    background-color: #1a75ff;
}

For the checkbox, you need this

[data-checkbox]:checked:before{
  background:#1a75ff;
  border:var(--border-size-s) solid #1a75ff;
}

.has-accessible-features [data-checkbox]:checked:before{
  border-color:#1a75ff;
}

The "#1a75ff" is the custom colour that I gave.

Kind Regards,

Márcio