Adding the class ‘listview-stylingless’ to your listview will remove the default mendix styling.
Btw, I would recommend to use a class for your custom styling rather then inline styling.
In addition to what Jean said, you can see some of the standard css classes you can use within Mendix (see under listview):
To your question on how to add custom styling:
You can create your own scss files with your own classes. These files can be importet to the custom.scss file in the sass section of your styling folder. After adding it, you need to compile the scss files to css using gulp, koala,…
Do not directly edit the css files. They will be overwritten when the scss files are compiled.