Where do the form button colours come from, editing variables.less does not have any effect? (Blank BS2)

I have an Acymailer module with a newsletter sign-up. It has a button and the colour is blue.

This button has a class btn-primary and that is defined in form.less

.btn-primary {

    .buttonBackground(@btnPrimaryBackgroundHighlight, @btnPrimaryBackground);

}

I have tracked down the value for @btnPrimaryBackground in variables.css like so:

@btnPrimaryBackground -> this is defined in -> @linkColor -> this is defined in -> @T3secondaryColor -> this is defined in -> @yellowDS -> and finally its value is: #ebb101;

But my button background is appearing as BLUE not YELLOW

However, if I edit the code above and directly include a yellow colour like this:

.buttonBackground(#ffd133, #ffc600)

Then the button is YELLOW!

So somewhere down the line the LESS processor seems to be ignoring the variable definitions.

Is this a bug?