Hi iankane2003 welcome to the forum
Short answer. You can't. It is more important for a browser user to have forms look consistent across sites, than it is for a site to have a form look consistent across browsers.
That said, forms can be styled so that they look acceptable, the chances are not so great that many visitors will be using multiple browsers, and of those that do, not all of them will care if even notice.
Of course if the form design is unacceptable in a certain browser, that should be addressed,
For example, say I want nice rounded corners. But a browser that doesn't support that gets square corners. It isn't all that bad is It?