Installing a theme is easy... Through Appearance=> install new theme...
On drupal.org you find many themes. Click right en copy the link. Choose the right version...
Paste the link into the textfield. The textfield beneath you use if the file is on your computer.
Set the theme active or as default.
Set the configuration ..
When you look at the block structure you can see the blocks provided in the theme.
It is usefull to know which blocks are available. Each theme has his own blocks.
The site branding is a block that contains the information of the site. The theme allows you to change some things..