Works with version 2.1.3 of MarkdownEditing.
You should have MardownEditing already installed.
a) Copy the file MarkdownEditor-Blackboard.tmTheme to the Packages folder.
Cmd+Shift+G to quickly go to folder by entering the path. The path is:
~/Library/Application Support/Sublime Text 3/Packages/User
Download the file, go to Packages folder, create a new folder for MarkdownEditing if one doesn't already exist and copy the file there:
b) Edit MarkdownEditing Settings to point to the custom theme.
Open MarkdownEditing settings from the
Preferences > Package Settings > Markdown Editing menu.
Alternatively, you can find the default settings files located at:
Packages/MarkdownEditing/Markdown.sublime-settings [GitHub flavored Markdown] Packages/MarkdownEditing/Markdown (Standard).sublime-settings Packages/MarkdownEditing/MultiMarkdown.sublime-settings
Foreground (text) #fff
Anchor Text #8da6ce