Which version of superset are you using? The easiest way to do so is using CSS. There is a preset link as well which you can refer to for fixing this particular issue as well.

For changing the CSS, click on edit dashboard and then navigate to the 3 dots to the top right part. Clicking on it will bring a drop down menu which you can then use edit the CSS by clicking the "Edit CSS" option. If you wish, you can save this css as a template to be used across other dashboards as well.