Try this code:
.rt-apollo-3 .sprocket-strips-effect-apollo:hover img {
opacity:1;
}
To create a custom CSS file, you just have to create a new css file at /wp-content/themes/[TEMPLATE]/css/ and name it [TEMPLATE]-custom.css. The new custom CSS file will automatically get loaded by the Gantry 4 Framework.
NOTE: The use of [TEMPLATE] in this guide is intended as a placeholder for the name of the template's primary directory. Replace [TEMPLATE] with the template directory name in order for these instructions to work with your site.
docs.gantry.org/gantry4/tutorials/adding-a-custom-stylesheet