The next/back buttons are changed via the button theme settings, although the images used are actually stored in a collection of images in a single "sprite" sheet file rather than a single button image. Each theme will have its own sprite sheet which defines the look of the different states of the navigation elements. Here's part of one from a Gomo theme.

The simplest way to change this in a project is to do the following: 

1. Publish and download your project (Actions/Publish/I want to download my project and publish myself)
2. Unzip the downloaded package
3. Open the resources folder
4. Find the sprite_navigation_desktop.png file and open it in PhotoShop or similar
5. Find the squares containing the elements you want to change e.g. Next - horizontal navigation
6. Modify them to your requirements, making sure that you keep them to the same size as the original
7. Save your changes
8. Go back to Gomo and upload your modified sprite file to a Shared Content folder
9. Now open your project, select the Settings/Theme tab and click the Customize theme settings button
10. Click the Buttons tab on the left-hand side
11. Find the Next and back section
12. Browse for the file you've just uploaded in the Default and Hover sections
13. Click the Apply changes button at the bottom right of the settings panel
14. Wait for the theme settings to be updated (the orange progress bar is shown at the top of the screen)
15. Now preview: your new next/back buttons should appear