UI theme settings

[MikeSullivan7]

Issue #101

This PR impliments RasCAL being displayed in Light or Dark mode depending on the users system settings. As default, this will be synchronised with the OS settings, but it is possible to override this be explicitly selecting Light or Dark mode in the Settings Dialog.

Things to test:

• Open RasCAL and check that it's appearance aligns with your OS Settings.
• Close RasCAL, change your OS appearence settings, reopen RasCAL and check that it opens in the correct mode.
• Open a Project and go to settings, change to opposite theme (e.g. Light -> Dark) and accept changes.
• Reopen RasCAL and check it has remembered the most recent theme change.
• Check that clicking "Reset to Defaults" in Settings changes the theme back to "system" mode.

[StephenNneji]

Thanks, works as advertised on Windows and MacOS, the theme does not update properly on Linux because of QT-BUG the problem is fixed in newer PyQT versions - 6.10.2 worked for me.

[StephenNneji]

Thanks for the update. Sorry I missed this when I last tested: If I change the theme to for example dark from light and I click the cancel button, it stays dark. I think it should revert to light we can easily modify the cancel button slot to reset, I tested this

def cancel_settings(self):
    if SETTINGS.style != self.settings.style:
        change_ui_style(SETTINGS.style)
    self.reject()

[MikeSullivan7]

Thanks for the update. Sorry I missed this when I last tested: If I change the theme to for example dark from light and I click the cancel button, it stays dark. I think it should revert to light we can easily modify the cancel button slot to reset, I tested this

def cancel_settings(self):
    if SETTINGS.style != self.settings.style:
        change_ui_style(SETTINGS.style)
    self.reject()

Thanks for this check, I thought I covered the cancelling part but I missed it. Sorted it now :)