anjalp/Minimalistic-Flat-Modern-GUI-Template ? reverse-engineered prompt

Build me a clean modern desktop app template in Python using PySide2, with a flat minimal look that works on Windows, Linux, and Mac. I want it set up so I can open it in Qt Designer and easily change the layout, colors, and widgets without fighting the code.

Please include a custom top bar with minimize, maximize, restore, and close buttons, a toggleable side menu, a dedicated About page, and a main window with a few stacked pages. Add the common UI pieces already wired up, like push buttons, progress bar, radio buttons, check boxes, text fields, sliders, scroll bars, combo box, and text browser. Also make a reusable custom dialog and error box where I can change the heading, message, and buttons.

Keep the style polished with a dual tone color scheme and a fully customizable stylesheet. If you need anything unclear, look up the current docs online and make it feel ready to run right away.