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/monochrome/icons/index.theme

@@ -0,0 +1,14 @@
+[Icon Theme]
+Name=monochrome
+Comment=Monochrome light icons
+Inherits=default
+Directories=16x16,48x48,256x256
+
+[16x16]
+Size=16
+
+[48x48]
+Size=48
+
+[256x256]
+Size=256