Following a suggestion of Kareninstructor here :

I created an highlighted textbox

For that, I use 3 new properties :

BorderColor : to set the color of the border
Borderwidth : to set the width of the border
Gradient : to say if you want a gradient of color for the border

The corners are not round yet (I am not very fond of it or maybe I am a bit lazy )

For anything else the textbox act as a common textbox :

Name:  HL-TB.jpg
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Here is a test application and the custom class :