I wanted to know if there is any way to customize the textbox's, for example: change the font of the written text, change the desainer. completely change it
You'd have to create it yourself. Use a Sprite or a 9-patch as the background/window for your text. Then place a Spritefont object with text on top of it to create the textbox-look seen in games.
The textbox itself is aesthetically very limited, it just displays the text with your chosen font.
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You can set the CSS style.
Oh, didn't know about that one yet. I stand corrected.
I think, the best way to create custom textbox is to make it via spritefont: TextBox via SpriteFont.