Styles are loaded in the order specified in the underlying code. It’s usually under control of the theme. If a 2nd style file has the same style defined, then the 2nd one overwrites the ‘parameters’ of the 1st style. This is as designed.
Note that it is not a good idea to edit a theme’s files directly. Any changes you make will be overwritten when that theme is updated.
If you want to customize a theme (and it’s styles), then it is best to create a ‘Child theme’. This is a theme that uses the coding/style from the ‘parent’ theme, then adds your customized stuff from the ‘child’ theme. This is a great way to experiment with theme customizations without the worry of your changes being ‘trashed’ when theme updates.
There are a ton of tutorials on how to make a child theme. I use a plugin called “Child Theme Wizard” to create the necessary child theme files.