First of all i would suggest having a program such as GIMP
http://www.gimp.org/ Be sure to create a back up of all the original textures.
Go to your Feralheart's "textures" folder and you will find a lot of images. The images that have the word "button, scroll, tab, text, name, target, title etc" are the ones you want to edit. I would start with one of the titles so when you open the game you can see it straight away without logging in.
Open one of the images up in GIMP and make a new layer (CTRL+L, there is a button in the layer window shaped as a piece of paper). When you get that options menu just click ok, as long as the fill type is on Transparency.
Have the new layer selected and change that layers mode to "Multiply". Now you cant paint over the images without losing the patterns and stuff in it.
When your finished, click Save As and go to your textures folder and replace the existing one.