Using Qt resources #10

- Introduced a QString variable `themeName` to store the current theme ("Dark" or "Light") based on the `darkTheme` boolean.
- Simplified setting icons for UI elements by using `themeName` to dynamically build icon paths, reducing code redundancy.
- Updated the logic for setting properties and icons in various methods (`MainWindow`, `on_startStopButton_clicked`, and `abort`) to use the new `themeName` approach.

- Simplified the logic for setting the theme button icon by using a
 single line of code with a ternary operator and string concatenation.
- Introduced a `themeName` variable to store "Dark" or "Light" based
 on the `darkTheme` setting, reducing redundancy in icon path
 construction.
- Removed redundant checks and streamlined the `on_themeButton_clicked`
 method for better readability and maintainability.
- Updated header file to include `themeName` as a private member
 variable.