So you probably want to know what your going to do to your game. Here's A screenshot of how I've configurated my game.
Upped bloom (quite a lot :P) and set shadows to unlimited range. I also made the UI 20% smaller and disable tooltips and tutorials.

So first go to the Besiege_data map, I you can't find this yourself, I advise you dont read any further.
In that map, there's a file called "config", select it, right click it and open it with noteblock (or Notepadd++ if available). It will give you this:
You can edit a lot of this stuff to tottaly get the UI to your taste, but be aware that some things can scrue over you game and ALWAYS make a BACKUP file.

Now you can enjoy the new look of your game!
Share this guide if it helped you, and of course check out my creations because a guide isn't a guide without self advertisement.

See ya