Rules Advanced Setting: CSS for Read More Button shortcode

Hi, In the Rules Advanced Settings, under Generated Post Content the default shortcut is:  %%item_content%%<br/>%%item_read_more_button%% After generating running and generating a post, the standard code returns as: Read More -------------------------------------------------------------------------------------------------------- My question is, where can I change the css code for the ? I don't want to use the default "button purchase" class, but rather my own custom css code.