Try re-installing the theme. You do this by visiting "Dashboard->Themes" and 'Remove'ing your theme. The theme will then show up again at the bottom of that page and you can re-install it.

You say you want to change the theme through each page's "Edit->Design" option. I'm not sure why you want to change the pages individually. To activate a new theme site-wide, visit "Dashboard->Themes" and "Inspect" your desired theme (to let C5 create any new page type files) and then 'Activate' the theme so it applies to all the pages in your site.

I tried removing all the themes, but the associated themes for my project (ourlitigator.com) still do not show up on my site. Please help!

Tell me more! It looks like you have removed all themes including the default C5 themes because your site can't find any theme css. When you say 'removing all themes', how did you do that? Through the C5 front end or did you delete the files on the server via FTP?

I deleted the themes through the C5 front end. Previously, although I associated all my themes (about 10) with the current project (ourlitigator.com), only 5 or 6 themes (3 default themes and 3 themes I purchased on C5) showed up on the list of themes. Thus, I deleted the themes one by one through the C5 front end, hoping they would show up from the bottom. No such luck. By the way, how do I edit the top "header" "Welcome to Kang Law Group on the web!!!"? Thanks in advance.

I logged into the site and here's what I needed to do to recover the missing themes. Visit 'Dashboard->Extend Concrete5' and un-install the themes one-by-one but don't remove the files. Then re-install the theme again and it should show up under 'Dashboard->Themes'.