Tumblr offers more than 50 themes in the "Grid" category, many of them premium themes that you must purchase before use. After you've installed a free or premium theme on Tumblr, you can edit the source code on your blog's customization page. To change the number of columns in a grid theme, you must know how to read and modify CSS stylesheets.
Modifying Various Themes
Open the customization page for your Tumblr blog and click the "Edit HTML" link to open up the source code for your theme. Every theme developer has a different style, and there's no uniform method for changing the column aspect of a grid theme. For instance, the "Astronaut" grid theme has a class called "#container .box". Changing the width in this class from the default "225px" to "150px" takes the theme from three columns to four. Some themes, like "Syndex, " don't require you to modify the CSS at all; the theme options allow you to set pixel width for post types, which changes how many display side-by-side. Not all grid themes can be easily modified. For instance, the "Club Monaco Theme" uses an externally linked stylesheet which you cannot edit.
Reverting to the Original Theme
If you decide that you don't like your modified version of your theme, you can go back to the original version. Open the "Browse Themes" section of the customization page. Select the original theme, click "Use" and save the changes to your blog.