Dia allows you to set a number of customization options that modify how the program works. These options are set by selecting the File->Preferences from the Toolbox Menu Bar. This opens the Preferences dialog. This dialog has tab panes grouping related preferences like: User Interface, Diagram Defaults, View Defaults and Grid Lines.
After you have made all the changes you want, click on to apply the changes and close the Preferences dialog. To cancel the changes and return to previous values, click the button. You may preview the effect of your changes using the Apply button.