Install theme on custom setting
On your dashboard click the customize link. Or paste this URL to your browser http://www.tumblr.com/customize/
On the menu bar select the “Theme”, the drop down menu will appear, and you can see the available theme ready to install. All you have to do is to click the theme and it’s now ready to use. Click save when you’re done.
Some theme is for sale and you can only use that theme when you buy it. You can buy it by depositing money and you can now have credit to buy the theme. It will be available to install after you buy it.
Using Custom HTML on custom setting
The html code of your current theme will show and you can freely edit it. It will be your advantage if you have knowledge of html and css. You can copy source code of theme available and paste it on your custom html. Click the update preview to display your new theme. Click save when you’re done.
Install theme using Theme Garden
Go to http://www.tumblr.com/themes/ and browse the theme that you like. Just like when you are in your custom setting but this time you have many more themes to select. You can browse for the Feature, Premium, Recent, and Popular Category of the theme. Premium theme is where you can buy and the rest is for free to use.
Preview the theme if you want to see how it is look like and click Install if you want to use it. You will be prompt where you want to install it if you have created more than one blog. Select the blog you want them to apply and click install theme.
Important! Changing theme can affect your previous setting on Appearance section. Every theme can have different setting, color, background, fonts, and different options. You can use this to feel and dress up your theme.
Note: If you want to see the basic setting of the theme, go to Appearance section and click the “Reset defaults”. This will shows the original appearance of the theme and then you can change it.