There are a few pieces of the puzzle with sliders. They come “pre-bundled”, as it were, with the Slider Wrapper (the main box that holds everything), the Mask, and then (by default) two Slides.
I’m not sure what you’re trying to center where but I’ll take a stab at this. All your content that ends up on a slide will be on the Slide itself, not on the Mask or controlled by the Slider Wrapper (my name for the main box that holds everything). For myself, I never control the content of an individual slide by changing the attributes of a Slide itself but rather I place a content wrapper (a div) on the slide and then I use the content wrapper to manipulate the content.
My thought is that since I’m likely to have multiple slides within the Slider component, they should all be the same and not manipulated at all. I literally never give them a class, attributes, or styling, that all happens within the content wrapper div (and maybe some positioning/sizing attributes on the Slider (main box). In other words, the Mask and Slides are “off limits” for changes.
I too “broke” a lot of Sliders before I figured out that I shouldn’t mess with the Slides. I use a lot of Flexbox (in general), and it works fine on the Slides as long as the content is in a wrapper that’s used as the parent for the start of the Flexbox items.
Hope this helps out in your situation.