Are you referring to the Featured Posts that appear directly under the grey navigation bar? If so, this feature does not require the CSS Upgrade. To activate this slider, all you need to do is set a few of your posts to be “sticky” and then make sure that they have Featured Images.
Please read the following links for more information on these topics:
As the second Support doc you were directed to says, “choose the image from your computer and upload it or choose it from one of the images already in your Media Library“. Featured images must be uploaded to your blog: you cannot use images stored elsewhere.