No particular laid out steps. It is a golden opportunity to learn using web developer extensions for your browser to experiment with your stylesheet(s).
A couple of things I would do first before diving in. Get the mod that better facilitates color changes for responsive_classic and install it. https://www.zen-cart.com/downloads.php?do=file&id=2088.
After it's installed and before you make any other changes, use the clone template mod to clone it to a new name for your store. Try to pick a clone name without the underscore. https://www.zen-cart.com/downloads.php?do=file&id=2087
Now, any changes to the new template can easily be found/removed/repaired using the old template and its clone.

For those of you wondering about underscores in URLs, Google, WooRank, and StackOverFlow all suggest dashes versus underscores.