Set this SHOW if you want to see the execution details and show debug details. Set this LOG if you want to write the execution details and debug details to the log file.
Select the depth of the details you need to see in the debug output.
Enable Template Debug setting if you need to check issues with a template in storefront and control panel. This will give you a link for storefront and admin with the debug parameter in the URL.
Almost all browsers have the Page or Code Inspector to examine the HTML and CSS of a page.
When you select some element on your page the Rules view lists show all the CSS rules that apply to the selected element. This can help you understand where CSS file located and what rule you need to edit.