Referencing stylesheet functionality for hover over using in portal forms

Dear SM community. Can I have your views over the following :

In our forms we have the possibility to use HTML code.

We would like to reference the stylesheet functionality to use hover over within our forms in our portal.
in our envir. we would like to use a grey-over for hovering, so the image gets a slight darker color.

As we would like to code it centrally and only use a div function withing the form.

Does anyone know where to find documentation on how to reference the stylesheet and then specifically hover over functionality?

Any hints are appreaciated.

Kind regards,
Patrick