"Keep it" and "Cancel" buttons from Fennec Add-ons Theme page regressed due to -moz-calc

[miketaylr]

See https://bugzilla.mozilla.org/show_bug.cgi?id=1334064

Describe the problem and steps to reproduce it:

1. go to mobile theme page
2. apply theme

What happened?

The "Keep it" and "Cancel" buttons are smaller in 53 Aurora and 54 Nightly than they are in release.

What did you expect to happen?

They're the same size.

Anything else we should know?

This is because -moz-calc was removed from Gecko and there isn't an unprefixed calc equivalent.

.persona-previewer .confirm-buttons .cancel {
    float: right;
    width: -moz-calc(50% - 39px);
}

should be

.persona-previewer .confirm-buttons .cancel {
    float: right;
    width: calc(50% - 39px);
}

etc.