The accepted answer here and other online resources suggest editing dotspacemacs-themes

dotspacemacs-themes '(solarized-light solarized-dark) 

I want to add two themes.

dotspacemacs-themes '(gruvbox sanityinc-tomorrow-eighties) 

On restart Spacemacs installs whichever theme is present on top. But after installation, if I change the order of themes, the new theme gets installed and previous theme for example "sanityinc-tomorrow-eighties" is lost.

I want to add themes such that when I do SpcTs (helm-themes) the theme appears in the list. Currently, only the top theme with other default themes appear in the list.

Edit:

I found Themes Megapack layer in official repository, which I think is relevant to my issue. But I don't understand how to use it. I suggests

To use this configuration layer, add it to your ~/.spacemacs.

Where inside ~/.spacemacs?