Ensure docs makefile downloads theme if failed before. (#3322)
The makefile did not download the theme if the directory "themes/gitea"
is there, even if empty.
On a fresh Ubuntu install, curl is not included, so the theme rule fails
just after creating the empty directory. When you try again after
installing curl, the rule is not triggered.
This could also happen if the download fails for other reasons.
This change makes the theme rule depend on the file "theme.toml"
which will be there only after unpacking a successfully downloaded
Signed-off-by: Alberto González Palomo <firstname.lastname@example.org>