Hello with your great helps and with the Cookbook Page 14 I could realize a Tooltip on Column-Headers in IG.
But this look not so good because Black&White and I use many pastell colours.
On the Interactive Grid in the initial Javascript Section I changed the code:
columnDef.jjs_helpTooltipText = "<h3>" + data.title + "</h3><p>" + data.helpText + "</p>";
To that:
columnDef.jjs_helpTooltipText = "<div style='background-color: rgb(235, 240, 229); color: black; font-size: 20px;'>"
+ $("<div/>").html(data.title).text() + ": "
+ data.helpText + "</div>";
But it is not really the end solution.

I do not know why the frame has a color of black maybe because I use the browser in Dark-Mode.
I would like to have two things:
1.) There is a black bold frame (maybe because of Dark-Mode), which I would like to be able to parameterize (change to the color see below)
2.) The data.title can contain some format styles because it is written always with different font size -> this is the reason why I wanted to extract only the Text.
The ID is too small and there is a big space between Title and text.
That would be the desired Tooltip:

Color: rgb(155, 180, 125) for the Frame.
Text in that:
"ID: Das ist die technische Schlüssel"
The ID should be underlined.
But it does not managed for me to solve it.
Could you please help me?