Applescript(Mail) modifies the text color if a document is attachet in dark mode

Since you are working in the default HTML behind the scenes, it will default to black text, and you need to designate a color for that text, or the words of that text:

use scripting additions

tell application "Mail"
	set msg to make new outgoing message with properties {visible:true, subject:"this email contains a wrong body font color"}
	set content of msg to "this text color will be modified"
	set color of words of msg to {65535, 0, 0}  # red text
	-- tell msg to make new attachment with properties {file name:POSIX file "/Users/anyUserName/anyFile.pdf" as alias}
	activate
	msg
end tell

When you are creating a new outgoing message in AppleScript, Mail has no clue about the current appearance mode (dark/light) and will default the text to black. That is fine when you have the appearance mode set to Light, but clearly not for Dark. Nothing you can do about that, other than color the text of the outgoing message, or do plain text.

Here is a StackOverflow link on how to use your AppleScript to tell mail to make a plain text message.

In AppleScript, when you are dealing with an application that has an AppleScript dictionary (Script Editor > Open Dictionary… > Mail), you are at the mercy of the application's AppleScript reserved words.

Although you can set the background color of an existing message, there is no means to set the background color of an outgoing message using the AppleScript dictionary for Apple Mail, without manually composing the outgoing message in HTML and using CSS for styling.