Many resource packs have a file located at assets/minecraft/textures/gui/widgets.png which results in the GUI looking different (including the hotbar). This mod instead uses widgets_original.png, meaning that changes to widgets.png don't show in the game with this mod installed.

To reproduce, load a texture pack like Sphax PureBDCraft, load into a world, and the hotbar texture will still have the vanilla texture.