Jeremy Herve on "[Plugin: Jetpack by WordPress.com] Where can I find the .slideshow-slide-caption div?"

I would recommend that you add your customizations to your own theme stylesheet, or to your custom CSS editor available under Appearance > Edit CSS in your dashboard. This way, your changes won't be lost when you update Jetpack.

If you'd like to edit an existing CSS property for that container, you do not need to edit the original CSS file: you can overwrite this property by being more specific. For example, you could make the caption red like so:

.jetpack-slideshow .slideshow-slide-caption {
	color: red;
}

I hope this helps.