[rofi-theme-selector] Replace $HOME with a tilde

[yusufaktepe]

This change will make it easier to share the configuration with different users.

[yusufaktepe]

@DaveDavenport I noticed that in 09e0f87, you preferred appending over changing the line. Is there a reason for this?

[DaveDavenport]

because sed modifying a @theme is not a valid way to completely change to a new theme.

This should be obvious:

@theme "mytheme"

@import "mytheme-mods"

window {
   fullscreen: true;
}