Most Feit Electric LED string lights are not dimmable or compatible with dimmers.
For the color-changing string lights, you may be able to adjust the brightness level using the included remote. You can look at replacing the bulbs in your string lights with dimmable bulbs.
For more information on string light bulb replacements read How to Replace String Light Bulbs.
The incandescent Feit Electric string lights are dimmable and can be used with dimmers. If the dimmer is outside it must be outdoor rated.
Here's a video with string light installation tips.