How to create UI -

1. Press "window".  Rename it to needed name of window. Move to any place, scale, rotate, assign color with color buttons,  move vertices of this plane to adjust window size, move pivot if you want. Type text in text field and press "assign command/text" and this text will appear in Help Line when your mouse pointer will be on empty space of window.

2. Press "button". Move, rotate, scale, assign color, move vertices, pivot..   duplicate, istance..   you can do ALL with buttons.  WARNING: You just can't do instance of groups!   To assign command type your command in text line, select button and press "assign command/text".  Buttons created with name "noname***"  - with such name button will be without name. If you want to label your button - just rename it to something that is not consist word "noname".

3. Press "text" to create text. Text it is like button, but it is text :). To assign text use text field and button "assign command/text". You always can assign color.

To adjust color - just adjust color of material(shader) of needed button-plane.

CODE OF YOUR WINDOW you can find in upper part of Script Editor. You can easyly edit it, replace buttons or text with some other ui-elements.

V.1.0.1  Little feature added - name of window polygonal plane now it is window name as you can see at picture (red is selection). Note for 1.0 versions scenes: delete your old window and create new without creating new scene (new version of window consist attribute "this is window", and old is don't)

Here you can see character animation control created ...  by 20 minutes..

This is example of CHAOS,  hierarchy not simple, buttons rotated, may be instanced, some of elements do not in window, but script also work!!!   but we can see some error here - buttonPlane on left must be blue, but it is green :(.    Use instances carefully,  very hard scene may not work correctly with materials or some other features.