Select the text, Insert/HTML..., enclose the text with the mark tag (see picture). Other colors can be defined with a span tag and a specific background color.

https://addons.thunderbird.net/en-US/thunderbird/addon/highlighter-222207/reviews/1161412/

too complicated just for one word...pls make it easy as I had in wlm!

Thanks sfhowes, that was easy instructions to follow and worked exactly as shown in your image.

I wrote text, then selected specific text with cursor as you would normally. Clicked on 'Insert' and select 'HTML' Then I just needed to type : <mark> before the text which was already auto inserted and then put the end the mark text after </mark> to indicate a stop highlight point then click on 'Insert' button. Job done. This automatically used a yellow background highlight.

I've never needed to do this before, but when the occasion arises, it's really useful to know a quick simple function.

ok ,it works but takes some time. by wlm took me a second, it was so simple

Feel free to use Windows Live Mail. It installs and works here in W10.

https://www.softpedia.com/get/Internet/E-mail/E-mail-Clients/Windows-Live-Mail.shtml

unfortunately, my joy was short. I chose only mail from the whole package. see my screenshot. i have w 10 pro. what do you advise me now to do?

See this discussion, which suggests you need to install the US English pack in Windows.