Improve dark mode support for editor and preview

[TeaTinyTool]

Summary

This improves dark mode support for the ACE editor and the file preview.

Problem

The TinyFileManager dark mode is currently not fully reflected in editor and preview views:

• The ACE editor always starts with its default TextMate theme.
• The file preview always loads the light Highlight.js style from $highlightjs_style.
• Plain text previews without Highlight.js do not receive a dark mode style.

Changes

• Use TextMate as the ACE theme in TinyFileManager light mode.
• Use Twilight as the ACE theme in TinyFileManager dark mode.
• Keep the existing manual ACE theme selector usable.
• Add a new $highlightjs_style_dark setting next to $highlightjs_style.
• Use $highlightjs_style in light mode.
• Use $highlightjs_style_dark in dark mode.
• Set the default dark Highlight.js style to vs2015.
• Apply a dark background and light text color to plain text previews without Highlight.js.

Expected behavior

• In light mode, editor and preview keep their current light appearance.
• In dark mode, ACE and file preview start with a matching dark appearance.
• Users can still manually change the ACE editor theme.