Notepad++ export to pdf or image and keep all formatting and colouring?

Use print functionality to do that. To get background color go to Settings -> Preferences -> Printing and check WYSIWYG. Also you need to have pdf or image printer. Google for one of them for ex. "pdf printer windows 10".

Edit: To make whole page color as background first remove text from header and footer in Settings->Preferences->Printing it will remove header and footer from print. Second use pdf/image printer options to scale and set position for your document. If there is no options for printer (for ex. win 10 pdf printer ) you can reprint pdf again using pdf reader scaling on print setting. But for pdf if you not set page color as background color you will always see a little white borders.

If you want to export it so that you can use it in a Word Document or something, I would suggest to copy-paste it in the Carbon app. https://carbon.now.sh You can choose among the theme dropdown to select a background/text colour or you can create your own theme by clicking + Create in the top option. If you wish to show the line numbers, go to Settings > Window and check line numbers You can then choose to export it by clicking the "export" button and click the PNG option in the Export as section.