The ten commandments to keep your hands off AndroWish
pdt I could be a bit picky with ttk, e.g. under Linux scrollbar buttons don't look great, treeview defaults to bold column titles. Under Windows, notebook tabs don't look as good as under Linux. I'm sure I could come up with some other minor points as well. But I don't think there's too much wrong really with how ttk looks.
The Linux tk file dialag leaves a lot to be desired. But I don't like the native Windows file dialog either. It's great if you like native, but what's native isn't always particularly good. I don't like the Microsoft ribbon much, and I don't like tool buttons in window title bars ...
Am I sounding grumpy? :) I won't start on Gnome3.
For cross-platform apps, attempting to look native is something I personally wouldn't go for. What I would go for is the app just must not be visually offensive.
I haven't used it in a while, but Synplify Pro always looked ugly to me, with its quite strong gradient shading on its toolbars and buttons.
tk under Linux looks bad. I used to write quite a bit of Java, and it's fairly easy to write an ugly Swing GUI as well, although with a bit of effort you can make them look just fine. Xilinx Vivado looks OK, I _think_ that's Swing ...
If ttk on Android can look as good as it does on Linux/Windows, great!