There are two ways you can change the color scheme in Houdini. You can change either the color scheme of all of Houdini or just the color scheme of the viewport.
Changing the color scheme of Houdini
Click Edit > Color Settings from the dropdown menu in the top left corner of Houdini. You can use the Color Scheme option to switch between a light, dark, or medium (Houdini Pro) color scheme for all of Houdini.
You can also click the Download button to open a list of color schemes which have been online posted by users.
For additional information, see the Color Settings help.
Changing the color scheme of the 3D viewport
Another way to change the color scheme of Houdini is to open the 3D Viewport Display Options dialog. Press D in the viewport to open the dialog, navigate to the Background Tab and change the Color Scheme dropdown menu.
This will only change the color scheme of the viewport, not all of Houdini.
For additional information, see the 3D Viewport Display Options help.