Using Themekey to configure a Theme, without changing your visible, default Theme
You can change your site to a different Theme, that only you can see, when logged into your account. This allows you to prepare a different Theme, while still displaying your current Theme to customers.
Choosing the Themes that you want to work with:
First, you'll need to visit the list of Themes, and make sure you enable any that you might want to use. Click "Appearance" in the top, Admin menu bar.
On the Theme page, your current, default Theme will be listed at the top of the page, along with any other Themes that have been enabled. Below that will be a list of the other, disabled, Themes. For any Theme that you want to have available to you, make sure you click the "Enable" link next to that Theme. You can enable as many Themes as you wish.
Configuring the Themekey tool:
Once you have enabled all of the desired Themes, you'll need to configure the Themekey tool. In the top, Admin menu bar, go to "Configuration > User Interface > Themekey > Settings > User Interface".
At the top, left of that page, click the link that says, "Selectable Themes". This will open a list of all the Themes that have been enabled, with a check box to the right of each one. Check each Theme that you want to be available in the Themekey tool (the "Seven" Theme is not necessary, as it is only used in the back-end of the site).
Once you've checked the Themes you want, you're ready to use the Themekey tool.
Selecting a Theme to display for your account:
Go to your own account page (Click your user name in the Admin menu bar), and click "Edit".
On the Edit page for your account, scroll down, looking on the left for a link that says, "Theme Configuration". Clicking the link will, again, display a list of all the Themes that have been enabled, with a radio button next to each.
On this page, you can select any one Theme to appear for you, while you're logged into your account. Once you select a Theme, and save your account settings, you will see the site in the selected Theme when you visit any customer-facing pages. This will allow you to configure Blocks, change colors (if the Theme allows it), and generally make sure the Theme looks just as you want.
To view the site in the default Theme again, go back to the Edit page for your account, and click the "Theme Configuration" link, and choose the first option at the top of the Theme list, "Don't switch the theme", and save.
To change the entire site over to the new Theme, click "Appearance" in the Admin menu bar. On the next page, click the "Set as Default" link next to the Theme you want to make visible to your customers, and click "Save" at the bottom.