Rework themes to easily use light/dark palette, using only different icons

- Renamed themes:
  - "colorful" to "default" and "colorful_dark" to "default_dark"
  - "default" to "monochrome" and "default_dark" to "monochrome_dark"
  - "colorful_midnight_blue" to "qdarkstyle_midnight_blue"
  - "qdarkstyle_midnight_blue" to "qdarkstyle_midnight_blue_monochrome"
  - qdarkstyle is renamed from "Dark" to "Mine Shaft" in the UI
- default and monochrome themes all use the same qss stylesheet
- Remove the ability to select "default_dark" directly
    - Default has better support for light and dark
    - Controller and Keyboard applets icons and style adapt to dark mode
- Add "qdarkstyle_monochrome" theme
- Remove duplicated icon files

dist/qt_themes/qdarkstyle_midnight_blue/icons/index.theme

@@ -1,14 +1,4 @@
 [Icon Theme]
 Name=qdarkstyle_midnight_blue
-Comment=dark theme
-Inherits=colorful
-Directories=16x16,48x48,256x256
-
-[16x16]
-Size=16
-
-[48x48]
-Size=48
-
-[256x256]
-Size=256
+Comment=Dark theme (Midnight Blue style)
+Inherits=default_dark