Editors : color themes

[JCBustamante]

May be a good idea to have independent color themes for different file types (e.g., latex, sage, .py, etc.). a nice dark theme for LaTeX editing leads to awful results in .sagews files.

[williamstein]

We now have an overall "dark mode" which seems to work well both in latex and sagews at the same time. This is in "Other settings"....