i am a happy user of DAT.gui for local debugging and controls of charts that i'm working on, but i need something that has basically the same functionality
Since dat.GUI already fulfills your needs your best bet is to stick with it. It's licensed under the Apache License V2 so you are free to modify it, make it into a closed-source project, etc. You can even release your new code under a completely different license (see Wiki for license comparisons).
As for changing the styling and adding features, there's nothing stopping you from adding your own CSS styles to override the defaults, or from adding your own js features.
Edit: Looking at the source, it appears the styles are set in the
style.scss file in the
gui directory. So you could edit this and re-build it, or just mess with the output css file itself and then re-compile the whole package (the source page has instructions for this).