At present (170426), although EasyEDA allows the export of a layer as `Black on White Background`, it does not support the export of a PCB copper layer as a negative (i.e. White on Black Background).
* This tip describes a workaround.
First, install a copy of the FOSS gerbv from:
https://sourceforge.net/projects/gerbv/files/gerbv/
* If you are running Linux then it is easiest to install gerbv from your distribution's repository.
1. Download and then unzip your Gerber file;
2. In gerbv, open the layer you want to negate (File > Open Layer(s));
3. Either press `F6` or, in the left hand panel, right-click on the selected layer and click on `Change color`;
4. Set the colour to `#FFFFFF` (white);
5. In the left hand panel, right-click on the selected layer and click on `Invert color`;
6. Then **EITHER** `File > Print` (CTRL+P);
7. **OR** `File > Export`, select the output file format then select where to save the file and enter the desired file name **including** the relevant suffix such as .png or .pdf.