To customize the appearance of text boxes:

1. Click the Themes tab.
   
2. Click the Edit or Customize Basic Theme icon.
   
3. Select the Body tab in the left pane.
   
4. Scroll down to the Text boxes section.
   
5. Click and drag the slider to adjust border width.
   
6. Click and drag the slider to adjust the roundness of the text boxes' corners.
   
7. Click the color grid to select the background color.
   
8. Click the second color grid to choose the font color.