A few suggestions to improve, IMO, the layout and functionality of the app.
- Settings: a little more space between the icons and the text, more space between entries
- Main window: more space between entries (if that option to expand space is selected), add the package origin country flag, more padding for the label
- Themes: if possible I would keep only "Light, Dark, Dark (OLED)" theme options but let the user choose his own color for the theme details. Or even better, let the user choose colors individually (background, headers, ...)
- Remove or tone down all the drop shadows, copy what most material based apps are doing so it doesn't stand out
- When you click on a package, allow the user to swipe right to go back to the main screen
- When you're checking a package details, the header is nice but I would change the layout below the courier (like China post in green) because the date and time (highlighted boxes) are a little too distracting. There's "Tracking ID" and "Created" on the header but the update time is aligned to the left, I would follow the same pattern and align it to the left.
Thanks, I hope you liked my feedback