[Background/Theme Color]

You can toggle the background color if you would like by clicking on "Vectorworks Preferences" in the Tools menu, then selecting the "Interactive" tab on the dialog box, and then pressing the "Interactive Appearance Settings" button at the bottom of the dialog box. Within these settings you can change the default colors of either the "white" or "black" backgrounds. 

[Can import dwg. to Vectorworks 2017 but can't edit]

When you import a single DWG/DXF, a dialog box should pop up. On the Primary Settings tab of this dialog box, make sure the "Reference" box at the bottom is unchecked. This should directly import the dwg and allow you to edit it. 

[Leader line on both sides to Callout Function]

You can! To add an additional leader line to the other side of a callout, you must right click on your callout and select "Add Leader Line." You can then drag this additional leader line wherever you'd like.