Hello @Odi-3, it should be noted that this request for integration is unlikely to proceed because there's no precedent of any other desktop environment's aesthetic guidelines being applied to the Steam client (cross-platform) and it's more likely that the Steam dev(s) want to keep the same look and feel across all OSes while allowing users the flexibility to use skins on their own.
Having written that, what I wrote doesn't automatically invalidate your request and we'll need to hear from a Steam dev if this request is worth considering at this time.
In case this feature is not possible, having the ability to use the system title bar could be a good compromise. This is already possible and default on Windows and MacOS. In Linux it could be optional with the addition of a specific setting.
@Odi-3, follow issues/1040 for that.
Integrating the Adwaita-for-Steam (https://github.com/tkashkin/Adwaita-for-Steam?tab=readme-ov-file) package into Steam's dependencies may allow you to add an option in your settings to make the Steam application more visually consistent with GNOME desktop environment.