macOS: add support for the new Dark mode

[mirams]

2021/07/01: it looks like dark mode (both on Windows and macOS) works better with Qt 5.12.x LTS (see here).

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The current LTS version of Qt (5.12.1) doesn't fully support macOS's Dark mode (e.g. see https://bugreports.qt.io/browse/QTBUG-73788 and https://bugreports.qt.io/browse/QTBUG-73852). Yet, there are things that we can still to improve things (note: some of those things were observed when starting OpenCOR in Light/Dark mode and then switching to Dark/Light mode and back):

• General:
  • Be able to detect the Dark mode
  • Change the colour of line widgets
  • Have context menus use the correct colour
  • Change the colour of links in QLabel objects ("dark" blue on a dark background doesn't look great; check the About OpenCOR dialog for instance)
  • Have Core::TabBarWidget behave properly in Dark mode (see the file/mode/view tabs in the central area)
  • Have Core::SplitterWidget use the right background
  • Have Core::CollapsibleWidget use the background (right now: white on grey background...!)
  • Have GraphPanelWidget::GraphPanelWidget always use a light surrounding background by default (since everything can be customised by the user)
• Splash screen:
  • Change its background colour
• Central view:
  • Change the background colour of our logo "view"
  • Change the background and foreground colours of "The XXX view does not support this type of file..." messages
• Help window:
  • Have the OpenCOR website always use a white background
• PMR window:
  • Change the background colour of the window when starting OpenCOR (it's white)
  • Change the colour of the PMR instance information (dark grey on dark background is not great) (This is actually the way it should look)
  • Change the colour of the "Filter" label
• PMR Workspaces window:
  • Change the background colour of the window when starting OpenCOR (it's white)
  • Change the colour of the PMR instance information (dark grey on dark background is not great) (This is actually the way it should look)
  • Change the background and foreground colours of messages (black on white in Light mode and currently white on white (!!) in Dark mode)
  • Change the colours of the diff dialog
• CellML Annotation view:
  • Change the colours used for the list of found and associated ontological terms
  • Use a dark background for the qualifier information page
• Text-based views:
  • Change the colours used for the line numbers area
  • Update the background colour when switching between Light and Dark modes
  • Make sure our different foreground colours work fine
• Simulation Experiment view:
  • Use the right colours for the runtime information and model type

Note: Qt's current support of macOS's Dark mode is rather limited. For example, one would expect QEvent::PaletteChange to be triggered every time we switch from Light to Dark mode (and vice versa), but it’s not the case. In other words, the best we can do is to get OpenCOR to look "ok" in Dark mode. However, to switch to Light mode while OpenCOR is running will result in the UI being completely messed up, but there is nothing we can do about it at this stage. (The same applies when starting in Light mode and then switching to Dark mode.) We therefore need to wait for Qt to "properly" handle macOS's Dark mode. Regarding QEvent::PaletteChange, we still handle it since this is what should be done if Qt was to properly support macOS's Dark mode.

---[ BELOW IS THE ORIGINAL MESSAGE FROM @mirams ]---

I have MacOS Mojave 'Dark Mode' switched on which is nice, but the Editing window looks like this (in snapshot 2019-02-01):

[agarny]

Nothing has been done in OpenCOR about macOS Mojave's new "Dark" theme, but I agree that we ought to support it, so tentatively putting it in for 0.7.

[agarny]

Having spent more time than I will ever want to admit on trying to get OpenCOR to look nice in Dark Mode, I have to admit "defeat". Qt is just not ready for Dark Mode despite QTBUG-68891. So, we are just going to enforce the use of Light Mode for now. Not something that I am happy about (not least because of all the time I wasted on this!), but too many other things to do, so...

[mirams]

Sorry Alan, enforcing light mode is fine! Thanks!!

[agarny]

Sorry Alan, enforcing light mode is fine! Thanks!!

No, it's me. I should have given up long ago. I just thought that I could fix all the tiny problems I came across, but some of my fixes had side-effects, etc. due, I believe, to Qt not being ready for Dark Mode.

Anyway, the master branch now contains (the couple of lines of) code that enforces Light Mode, so I will generate a new snapshot before the end of the week.

[agarny]

When we first looked at this issue, we were using Qt 5.12.1. Right now, OpenCOR uses Qt 5.12.11 (definitely not the latest version of Qt, but this is unfortunately the last real LTS version) and it looks like dark mode support is quite good. So, we might be able to get it done this time.