You can choose a different theme from the Add-on Settings area. Go to the Extensions page and click on Settings for Debug Toolbar.
Select the theme you want the Toolbar to use. EEDT ships with 3 themes (Default, Bootstrap, and Hot Dog Stand).
Where do you want the toolbar to appear on the page? (Top Left, Bottom Left, Top Right, Bottom Right)
You can also use Config Overrides to set the settings for the toolbar.
The convention for Focus Lab LLC Master Config:
<?php
$env_config['eedt']['theme'] = 'default';
or
<?php
$env_config['eedt']['theme'] = 'bootstrap';
The convention for EE Config:
<?php
$config['eedt']['theme'] = 'default';
or
<?php
$config['eedt']['theme'] = 'bootstrap';