Note editor: font color and background color of source code blocks should be consistent with used color theme

[d1vanov]

The background color for source code blocks within the note editor is now hardcoded. It doesn't seem to be well compatible with font color inherited by the note editor from the theme: when dark theme is installed for the desktop, background color is typically dark and font color is light. Source code blocks' background color is already light and it leads to bad visibility of text within it.

In the ideal world both background and font colors of source code blocks would be computable from theme's colors but it won't be easy to do. What would be easy to do for a short term fix is to hardcode both background and font color within source code blocks.