Downloading the theme from its homepage and then using "Install From File..." works as expected.

Can confirm, this one does not download for me either with Frameworks from git master.

Huh, works perfect for me from master.

Can you please enable the debug messages for knewstuff (kdebugsettings is a GUI tool).

Then start the kcm from the command line
>kcmshell5 kcm_sddm

And install an entry, when the password dialog pops up check if there is output like:
>kf.newstuff.core: Install to file  "/tmp/Lime-Green.tar.gz"
>kf.newstuff.core: move:  "/tmp/Lime-Green.tar.gz"  to  "/tmp/Lime-Green.tar.gz"
>kf.newstuff.core: Run command:  "sddmthemeinstaller -i /tmp/Lime-Green.tar.gz"

(In reply to Alexander Lohnau from comment #3)
> Huh, works perfect for me from master.
> 
> Can you please enable the debug messages for knewstuff (kdebugsettings is a
> GUI tool).
> 
> Then start the kcm from the command line
> >kcmshell5 kcm_sddm
> 
> And install an entry, when the password dialog pops up check if there is
> output like:
> >kf.newstuff.core: Install to file  "/tmp/Lime-Green.tar.gz"
> >kf.newstuff.core: move:  "/tmp/Lime-Green.tar.gz"  to  "/tmp/Lime-Green.tar.gz"
> >kf.newstuff.core: Run command:  "sddmthemeinstaller -i /tmp/Lime-Green.tar.gz"

Got an interesting error dialog ("Unable to install theme") after waiting at the password prompt for a few seconds:

> DBus Backend error: could not contact the helper. Connection error: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.. Message error: 

Debug output:

org.kde.knewstuff.core: Install  "Yellow"  from:  "https://api.kde-look.org/ocs/v1/"
org.kde.knewstuff.core: Link ID already known 1
org.kde.knewstuff.core:  link for  "1309322"

org.kde.knewstuff.core: Downloading payload QUrl("https://dllb2.pling.com/api/files/download/j/eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6IjE1NjAyMzkxNDkiLCJvIjoiMSIsInMiOiJkZjA1MjUxMzg3NTgxYjFiYjE1ZmM5MDJiNGVhNjliODRmMWY1OWE1NGE3NDIzM2M2YzhiYTIyYjdkOGVkNGEzYjEzNWFmODYwM2RlY2M3OWM5NjFhZDhmMTQ0ODc4YTlkMGUyZGEwODc0MjA5ZmVkODdkMDNhMzZiOWZjZmZkMCIsInQiOjE2MDAxODE5NjAsInN0ZnAiOm51bGwsInN0aXAiOiIyMTIuMTI5Ljg3LjgxIn0.CsPwe65TP7wi26kHe9lQQdWGwbsmjo1E8DPZ8UKu9Bo/Yellow.tar.gz") to QUrl("file:///tmp/yRKvQN-Yellow.tar.gz")

org.kde.knewstuff.core: File copy job is from (or to) a remote URL
org.kde.knewstuff.core: virtual void KNSCore::DownloadJob::start()
org.kde.knewstuff.core: KNSCore::HTTPWorker::HTTPWorker(const QUrl&, const QUrl&, KNSCore::HTTPWorker::JobType, QObject*)
org.kde.knewstuff.core: Opened file "/tmp/yRKvQN-Yellow.tar.gz" for writing.
org.kde.knewstuff.core: Writing 3426 bytes of data to "/tmp/yRKvQN-Yellow.tar.gz"
org.kde.knewstuff.core: Wrote 3426 bytes. File is now size 3426

[...]

org.kde.knewstuff.core: Write registry

[...]

org.kde.knewstuff.core: void KNSCore::HTTPWorker::handleFinished() QUrl("https://dllb2.pling.com/api/files/download/j/eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6IjE1NjAyMzkxNDkiLCJvIjoiMSIsInMiOiJkZjA1MjUxMzg3NTgxYjFiYjE1ZmM5MDJiNGVhNjliODRmMWY1OWE1NGE3NDIzM2M2YzhiYTIyYjdkOGVkNGEzYjEzNWFmODYwM2RlY2M3OWM5NjFhZDhmMTQ0ODc4YTlkMGUyZGEwODc0MjA5ZmVkODdkMDNhMzZiOWZjZmZkMCIsInQiOjE2MDAxODE5NjAsInN0ZnAiOm51bGwsInN0aXAiOiIyMTIuMTI5Ljg3LjgxIn0.CsPwe65TP7wi26kHe9lQQdWGwbsmjo1E8DPZ8UKu9Bo/Yellow.tar.gz")
org.kde.knewstuff.core: Data for "https://dllb2.pling.com/api/files/download/j/eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6IjE1NjAyMzkxNDkiLCJvIjoiMSIsInMiOiJkZjA1MjUxMzg3NTgxYjFiYjE1ZmM5MDJiNGVhNjliODRmMWY1OWE1NGE3NDIzM2M2YzhiYTIyYjdkOGVkNGEzYjEzNWFmODYwM2RlY2M3OWM5NjFhZDhmMTQ0ODc4YTlkMGUyZGEwODc0MjA5ZmVkODdkMDNhMzZiOWZjZmZkMCIsInQiOjE2MDAxODE5NjAsInN0ZnAiOm51bGwsInN0aXAiOiIyMTIuMTI5Ljg3LjgxIn0.CsPwe65TP7wi26kHe9lQQdWGwbsmjo1E8DPZ8UKu9Bo/Yellow.tar.gz" was fetched "(NOT cached)"
org.kde.knewstuff.core: Install:  "Yellow"  from  "/tmp/yRKvQN-Yellow.tar.gz"
org.kde.knewstuff.core: installdir:  "/tmp"
org.kde.knewstuff.core: isarchive:  true
org.kde.knewstuff.core: installing non-archive from  "https://dllb2.pling.com/api/files/download/j/eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6IjE1NjAyMzkxNDkiLCJvIjoiMSIsInMiOiJkZjA1MjUxMzg3NTgxYjFiYjE1ZmM5MDJiNGVhNjliODRmMWY1OWE1NGE3NDIzM2M2YzhiYTIyYjdkOGVkNGEzYjEzNWFmODYwM2RlY2M3OWM5NjFhZDhmMTQ0ODc4YTlkMGUyZGEwODc0MjA5ZmVkODdkMDNhMzZiOWZjZmZkMCIsInQiOjE2MDAxODE5NjAsInN0ZnAiOm51bGwsInN0aXAiOiIyMTIuMTI5Ljg3LjgxIn0.CsPwe65TP7wi26kHe9lQQdWGwbsmjo1E8DPZ8UKu9Bo/Yellow.tar.gz"
org.kde.knewstuff.core: Install to file  "/tmp/Yellow.tar.gz"
org.kde.knewstuff.core: move:  "/tmp/Yellow.tar.gz"  to  "/tmp/Yellow.tar.gz"
org.kde.knewstuff.core: Run command:  "sddmthemeinstaller -i /tmp/Yellow.tar.gz"

I also noted that it exists after waiting too long for the password input. But does it work when you type the password in normally?

It might also be useful if you could run the install command directly:

>sddmthemeinstaller -i /tmp/Yellow.tar.gz

(In reply to Alexander Lohnau from comment #5)
> I also noted that it exists after waiting too long for the password input.
> But does it work when you type the password in normally?
> 
> It might also be useful if you could run the install command directly:
> 
> >sddmthemeinstaller -i /tmp/Yellow.tar.gz

Well, I swear it didn't work (at least when using the Lime-Green thene) when I tried it multiple times yesterday, including timely entry of the password. I just now tried again with the Yellow theme, and it worked and appeared in the gallery.

Odd.

I've just tested 4 themes from the store and each was installed without issue. Are you still able to replicate this issue?

(In reply to Justin Zobel from comment #7)
> I've just tested 4 themes from the store and each was installed without
> issue. Are you still able to replicate this issue?

Hey, sorry I'm not on KDE / Plasma anymore, but I assume it's fixed by now :) Thanks for your efforts.