To change the colour of an item
Click on the item that you want to change and you'll see it's name appear in the listbox at the bottom. Then click on any of the coloured squares, and you'll see the colour of the text for that item change accordingly.
To change the background colour
Just click on the background itself and select a colour as usual. You can also click on the background of the editbox and the listbox to change their colours as well.
If you don't like the changes you've made
You can either click cancel or you can click the reset button which will reset the colours back to the default mIRC colours.
Other people might have a background colour that's different from yours, so what appears clearly on your background colour might be hard to read on someone
else's, or vice versa. For example if your background
color is red, and someone uses red text, you will not
be able to read the text.