Have you tried to contact the developer to see if they know how to do this.  You can use the console to show what css changes that text color, then add that css in your admin theme custom css so it does not get overwritten by an update. 

Yes I did.
no reply from developer
this is why I'm trying here

I have tried to look what change it.  I simply could not find it

what browser are you using chrome or FF ?

I will assume you are using FF 

just open up the message box, right click and choose inspect element

the code is below, as you move up and down on the code the above will show you what is affected.  

If you look on the right side below, it shows you the css you may have to play with it a bit to get so you are familar with it. 

if you click on the color dot on the right side there you can change the color and see if it changes in the box.  So this is the css you need to put in your custom css in admin themes css

.htmlarea_styles_mailbox{

color: rgb(51, 204, 102);

}

and that will make that text green.  If you want another color just change the color on the popup and it will show you the code. 

Dont forget to clear your cache with dev mode 

Actually its not working, wait till tonight please darryl is good at css im not, lets see what he says.. sorry 

actually wait, im using cloudflare too and i didnt clear my cloudflare cache.  So it should work for you if you dont use cloudflare. 

This should take care of the subject line, and the message window. The dev must have set the color to #fff. Just copy, and paste this into you custom css box for the theme in the admin panel.

.ow_chat_dialog .ow_chat_subject_block input {
    color: #585858 !important;
}
.htmlarea_styles.mailbox {
    color: rgb(51, 51, 51) !important;
}

I was almost right. The dev did add a light font, but should have changed the color scheme of the message box to accommodate the font color. They also used the "!mportant" tag to override the ow css. This should work for you.

I didn't see any posts in the theme's forum. You may want to bring this to the devs attention so he can address it with an update as well.

Thanks Darryl for the help, excellent. :)

Hey Darryl.

I have copy-paste the code to admin panel
nothing happen.

Taking your advice and reposting this issue at Theme Forum.