Form Lightbox button is "invisible"

Hi! I have created forms on a website and want to use the Lightbox design. My problem is that the send-button at the bottom of the form is "invisible" - the text is white as well as the outline of the button. It is only when I hover over the place where the button is that the text send (skicka in Swedish) appears. You can see it on the pictures. I have tried to customize the buttons in different ways and change colors but I can't seem to change this particular button and frankly I have no idea where to change this so I have really just tried randomly. The problem is only when I use the Lightbox design, not when the Lightbox-mode is turned off. Can anyone help me?

Can you share link to page where you use form?

Or try adding this to Design > Custom CSS

.lightbox-inner input.button {
	color: #f1f !important;
	border-color: #f1f !important;
}

Hi! Thank you. You can see it on this webpage: https://www.sweboat.se/lankar

I will try the code you provided. 

Thank you so much!