The highlight color is theme related and cannot be changed unless you download Plazmic theme builder and create a new theme or find a theme made by someone else with a highlight color that you like.
If there is an app out there that will do it for you I am not aware of it, but apologize if there is such a thing...
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Thank you. I notice that the background color does change according to the theme but is still largely blue when I'm typing a text message, an e-mail, etc., the background highlight color is blue. I have attached two screen captures for reference.