When you move the pointer over the edge of a div tag in Design view, Dreamweaver highlights the borders of the tag. You can enable or disable highlighting as necessary, or change the highlight color in the Preferences dialog box.
To change the highlighting color for div tags, click the Mouse-Over color box, and then select a highlight color using the color picker (or enter the hexadecimal value for the highlight color in the text box).
To enable or disable highlighting for div tags, select or deselect the Show checkbox for Mouse-Over.