The user interface looks pretty nice, though there are some things I would like to see changed:
The two buttons on the welcome page are not properly padded. I would suggest you spacing them down a bit from the blue area of the page.
Some text labels, mainly the titles of the pages, look off when placed at the very edge of the border. I suggest you space them slightly inwards so that it looks more aesthetically pleasing.
Also, since you know this stuff, Im relatively new to scripting, But i’ve been for a long time, trying to make a working Admin GUi, Like yours, would you be willing to tell me a little bit on how you did your scripts for the ban & kick Buttons?
I would add rounded corners on all the UI using UIcorner or Roundify. Maybe the hovering changing colors instead? What I mean my style matching is that not all the text is centered, and I would add a little more spacing with the buttons.