Its not possible to provide all the elements to be available to modify from the admin panel from a input. Sometimes users need to change the style of some certain elements that are not available to change from the admin panel. For that you can use custom css. To use custom css you can follow the steps given below.
1.1st go to your site right-click on your mouse you will find an option inspect element.
2. After clicking inspect element you will see a new window below with the HTML code and the CSS codes for the site.
3. If you click this button from the screenshot
4. Than when you take the mouse pointer over the element you want to give custom css to will be highlighted
5. After clicking that you will get the CSS for the element. Which you can edit to your preference give it a size or give its own colour.
6. And lastly, copy the CSS code and paste them to Appearance > Customize > Additional CSS
