set_custom_style()

Mapping:
  - "logical name" (the one used by the backend) used as a fallback if there isn't any mapping or if the mapped file isn't found
  - done by namespace:
    - The global namespace (__main__ is always a copy of an existing namepsace => the mapping is also copied)
    - The 'core' namespace uses the mapping of the user style
    - The '<style_name>' namespace uses the mapping of the style
    - The '<vendor_ext_name>' namespace uses the mapping of the user style
  - the mapping of a style is the combination of its mapping file and of all its parents (the oldest parent is merged first, so the more specific can override it)
  
Namespaces:
  - for the classic pages (index/viewforum/faq etc...)
    	@__main__ :
      - styles/style1
      - styles/style2
      - styles/style3
      @style1
      - styles/style1
      - styles/style2
      - styles/style3
      @style2
      - styles/style2
      - styles/style3
      @style3
      - styles/style3
      @ext1:
      - styles/style1/ext/vendor/ext1/
      - styles/style2/ext/vendor/ext1/
      - styles/style3/ext/vendor/ext1/
      - ext/ext1/styles/style1
      - ext/ext1/styles/style2
      - ext/ext1/styles/style3
      - ext/ext1/styles/all
      - styles/style1
      - styles/style2
      - styles/style3
      @ext2:
      - styles/style1/ext/vendor/ext2
      - styles/style2/ext/vendor/ext2
      - styles/style3/ext/vendor/ext2
      - ext/ext2/styles/style1
      - ext/ext2/styles/style2
      - ext/ext2/styles/style3
      - ext/ext2/styles/all
      - styles/style1
      - styles/style2
      - styles/style3
      @ext3:
      - styles/style1/ext/vendor/ext3
      - styles/style2/ext/vendor/ext3
      - styles/style3/ext/vendor/ext3
      - ext/ext3/styles/style1
      - ext/ext3/styles/style2
      - ext/ext3/styles/style3
      - ext/ext3/styles/all
      - styles/style1
      - styles/style2
      - styles/style3