i have installed 12.5.455, by default all objects appears black in my viewport. Is this a bug or it needs some change in some setting ?

Looks like you have Intel graphics and are using OpenGL 2 for the viewport display.

Go to menus-> Edit/Preferences/3dViewport -> set Scene Renderer to H11 or GL 1.2
Click Accept and the you should see a shaded sphere.

Houdini doesn't support Intel graphics fully.
http://www.sidefx.com/index.php?option=com_content&task=view&id=415&Itemid=269 [sidefx.com]

There have been a few OSX versions where the gl_FrontFacing GLSL builtin was being incorrectly set for certain hardware (always false). What OSX version and graphics hardware do you have?

Have you got a light in the scene which is on the object? I can see from your image that at the moment you've got the “Normal Lighting” option turned on and if you don't have a light in the scene, which is on the object, then it will be black.

To fix you should just be able to create a light in the scene which is on the object or switching to Headlight only mode (On the toolbar running vertically down the right hand side of the 3d viewport it's the 4th button down with the arrow pointing to the sphere - at the moment you're using the one which is the 5th one down and has 2 spotlights on the sphere)

Cheers

Tony