AutoCAD Mechanical toolset ships with a dark background color.
If you open a drawing that was created under a light background, the object colors in that drawing may not contrast well with the dark background. In such a case, you must visit the Object Property Settings dialog box and change the colors of these objects.
If you have not customized object properties such as linetype, lineweight layer and usage in hide, you can use the Restore Defaults button to automatically change the colors of the objects. This button intelligently restores the correct color schemes depending on the background color.
If required, you can revert the background color to black. In this case, you must change the path to the templates so that AutoCAD Mechanical toolset picks up the dark background templates by default. The dark background templates are stored in a folder named Darkbackground, which is a sub folder of the default templates folder.