1.Go into your main FeralHeart folder, which should be in OS(C:).
2. Go to media, then textures.
3. Find the texture you want to be changed, in this case search for the textArea and textField ones. There are others, too. You can change the tabs and the buttons (which would be at the top in the folder, if it's in alphabetical order).
4. Open the texture in GIMP, Photoshop, Paint, or even Paint Tool SAI. Whichever photo editing program that you have should be fine.
5. Look for an option in your program that could add a hue or colored filter to the image. From there, choose the color you want it to be.
6. Save, and make sure that you save it in the textures folder where it was before.
Then, your FeralHeart interface should have a changed color in it next time you log on. I hope this helps!