Yes, for your own theme you will need to provide your own css for galley and slider styling. Have a look at how Elemental or Atomik style the blocks, either by reading code or creating a new site to experiment with and using the browser dev console.
There are also many free and paid slider and gallery addons in the marketplace, some of which provide all their own styling. For example, my own Omni Gallery is independent of front-end theme. Omni Gallery - Concrete CMS