What is a CSS?
CSS(Cascading Style Sheets) refers to a type of website stylesheet that instructs your web page to perform in a certain way. You can not only decorate your HTML elements, but also create a modal, layout, gallery, or even in-page animations with CSS.
Why do I need this?
Custom styles are very important when it comes to a personal, or business website. If you are using a web template for your blog, or website, you may want to go beyond what your template options provide. CSS does the job for this.
Can I add my CSS to Tumblr?
Of course! You can place a inline CSS, or file on your theme code. Tumblr also provides a useful tool that enables you to place your custom CSS without changing any theme code.
Using custom CSS
Open up your customize page. Click 'Advanced options' on your blog.
On your advanced option page, you will be able to find a custom CSS input. Put your any custom CSS in this inbox.
And, you are good to go! Feel free to leave a comment if you have any questions!