Evaluate the use of the Basic Style and adding a Custom Style

There were some changes in Qt, so we should re-evaluate switching to a custom style. Idea:

• Switch to using the Basic style as a base.
• Introduce a custom theme (and handle any custom styling there).
• Define a new, consistent color scheme (both for Light and Dark settings).
• Allow tweaking the colours, as requested via #611 (closed).
• Consider updating the Icon to match the new color theme.
• Update the user manual to follow the new color scheme.

References:

• https://doc.qt.io/qt-6.2/qtquickcontrols2-customize.html#creating-a-custom-style