I can understand .region-sidebar is a class defined in the Atlas Default layout but I couldnt fidn where these classes like .mx-icon-filled , mx-scrollcontainer-open:not,.mx-scrollcontainer-wrapper are assigned !!
Hello,
You can use VSCode "find in files" tool to see where these CSS classes are defined:
When you open the developer tools of your browser, you can see where the CSS is defined:
But be aware that altering these files is not recommended.
Use the custom styling of Mendix to overwrite the default styling of Atlas.
More info here: https://docs.mendix.com/howto/front-end/customize-styling-new/
Hope this helps!