Your theme is how your blog looks at its web address on a regular computer (see ours at staff.tumblr.com). You can choose from tons of themes at tumblr.com/themes, and you can always edit your theme from the Customize menu.
You can check out any blog’s theme (including your own) by visiting its URL directly, or by clicking on that blog in the dashboard, then clicking the username at the top of the panel that slides out.
If you want to edit the HTML of your theme, use the editor in the Customize menu of your blog. The Description box is also HTML-enabled, if you just need a place to put small snippets of code.
If you want to know all the particulars of Tumblr’s theme code, have a look at our guide.