KickstartCassiopeia has the option to add custom css without the need for a custom css file or ftp access.
You can simply add css code in the module, and it will be the last code called, so you can always override a previous css entry. To do this, simply go to the custom CSS tab, and add your css. You are already within a css entry, so you only need to call the classes or ID's you need to change en enter them here. Usually an !important statement is not required. Not always!
Check below for an example:
You can now add custom css for all viewports, a mobile viewport and a specific desktop viewport. This enables you to specifically add css to whatever responsive view you desire.