Tumblr lets users customize HTML and CSS elements inside a theme's template. Some themes with an "Install Now" or "Install this Theme" button that lets other Tumblr users easily install a theme used by a Tumblr blog that they like. You can't completely remove this button, but if you'd prefer this button not to display on your site, you can easily hide it using basic CSS.
Sign in to your Tumblr dashboard and click on the "Gear" icon.
Click on "Customize Your Blog." A customization box is visible in the left pane. Your theme is listed at the top. Click on "Edit HTML" next to your theme. The template file opens.
Browse through the template file until you find the CSS code that generates the "Install" button. It should start with a declaration named "Install" or "Install Button." Beneath it will be the properties for the button. You'll see the following code:display: block;
Change "block" to "none." The code now looks like this:display: none;
Click on "Update Preview," and then click on "Save."