Will it be possible to mod the UI? I am a UI design specialist and would love to put those skills to use in Automation. Even if it’s just replacing image files and colours, that’s fine.
We’ll be updating the UI when / if we switch the game engine. How moddable it will be is not known yet though 
What you really need to do is allow windows in game to be resised and minimised and maximised. So changing game options like sound levels in game is possible without closing all the game windows. This I hope is priority #1 when revamping the UI.
In the latest ‘Little Dev Update’ video, Killrob showcases how the revamped UI looks, and it has been confirmed that the game will no longer be played using separate in-game windows for the separate designers. Instead, there will be a menu along the bottom to navigate to the separate stages of your car/engine design.
I think you will still need to go back to the main menu to change sound options, but the game will save continually (like Google Drive for example) so you can go back and change options without losing your current design.