How can I changed the colors?

Sarah Gooding writes:

The design is generic enough to drop nicely into any theme, but you can further customize it by copying the file from the plugin’s /template-ideas/ directory into your theme. Idea Factory’s settings also allow you to disable the core CSS file from loading so that you can take full control.

Source: https://wptavern.com/idea-factory-a-frontend-submission-and-voting-system-for-wordpress