Hi,
You can set the black background on the page itself. To set that: Page
-> Properties -> Background tab.
You can also set the black background on shapes. To set globally, the
easy way is to edit the "Graphic" style. Format -> Styles -> Manage
Styles, right-click on the Graphic style, select Modify. Then change
the background color to black.
For the charactor color, if it is set to Automatic, it should change to
white by itself. If you want some characters to be red, then highlight
the letters and set the color in Format -> Character -> Font Effects.
When you export, select what you want to export (CTRL-A for all or Edit
-> Select All) and then in the bottom left corner of the file selection
dialog, select the "Selection" box. This will make the exported image
exactly the same size as your drawing. Otherwise, the drawing will have
the size of the page which will make it more difficult to adjust if you
bring your PNG in another document.
I hope this helps.
Rémy.