You need to import custom textures to replace the current loading bar. What part do you want to change? Loading bar has these 5 textures:
UI\Glues\Loading\LoadBar\Loading-BarBackground.blp
UI\Glues\Loading\LoadBar\Loading-BarBorder.blp
UI\Glues\Loading\LoadBar\Loading-BarFill.blp
UI\Glues\Loading\LoadBar\Loading-BarGlass.blp
UI\Glues\Loading\LoadBar\Loading-BarGlow.blp