If you subscribe to an optin, the button briefly shows a white animated gif. You don't see that if you use a white button. To fix that, upload an animated gif in a different color to the Bloom folder. And remember to do that after each update.

/wordpress/wp-content/plugins/bloom/images/subscribe-loader.gif

You can't use this CSS because the real source css already uses !important

.et_bloom .et_bloom_subscribe_loader {
display: none;
background:
url(//www.horecawebservice.nl/wordpress/wp-content/plugins/bloom/images/subscribe-loader.gif)
!important;
}
Code language: CSS (css)

To change a color in the animated gif you can use:

• <http://www.gifmagic.com/> Effects » color mods » replace color