There is a big caveat about the internal editor: syntax highlighting is not customizable by skins. Secondly, the syntax files shipped with midnight commander use ANSI colors, so unless you configured your terminal to use the solarized color palette for the 16 standard colors, the colors used for syntax-highlighted elements might not be from the solarized palette at all.
Besides the non-deterministic colors resulting from the use of ANSI colors, the worst thing about the inability to customize the syntax highlighting is that some of the colors are barely readable depending on the background used by the skin. You can see this in the below screenshots in the light variant for the window that shows README.md. The markdown syntax highlighting uses "lightgray" as the text color, which is ANSI color "white". In solarized light, this color is used for the emphasized background. Used as foreground color on the standard solarized light background, it is barely readable. There is a longstanding related issue concluding that light background skins are problematic with the current syntax highlighting.
In the dark variant, the colors seem to work better but there may be problems with some filetypes as well that I did not encounter yet.
As a workaround other than using an external editor or disabling syntax highlighting, the makeskins.pl in this
repository has the ability to generate an adapted syntax file that uses a) truecolor / 256color definitions and b)
attempts to remap the grey tone colors (used in solarized for foreground and background colors) according to light or
dark skin. I found this to work reasonably well, so far, as to it produces readable results. Of course, if the syntax
files use accent colors for background, this is what you will still get, but it is normally only used to highlight small
parts of the file like individual keywords.
Below shows screenshots with the standard syntax files as well as with those generated by makeskins.pl.
Highlighted entries of the history dialog unfortunately use the color of focused hotkeys. This does fit the theme, but cannot be avoided currently. See ticket.
