Add Markdown UDL of dark mode and the ability to apply it:
After dark mode being swiched in, all files with extension .md will be opened by applying Markdown UDL of dark mode.
When light mode is switched back, the default Markdown UDL (of light mode) will be applied to the new opened .md file.
Please note that all .md files opened before switching to new mode, will remain in the previous mode. User has to close and reopen these files to have the right Markdown UDL applied.
Fix#10196, close#10347
Don Ho