Error adding sddm theme.

[Refpga]

Hello, I'm trying to apply an sddm theme (called slice:
https://github.com/RadRussianRus/sddm-slice). The version installed on
my system is aaw7i330kq9pc98dwaj8zncz096izav0-sddm-0.18.1.
My sddm-configuration contains the following lines:

(theme "slice")
(themes-directory "/home/username/.guix/sddm-themes")

And path to the theme directory slice is
/home/username/.guix/sddm-themes/slice. Moreover, Main.qml exists in
that directory, with permissions -rw-r--r-- (644).

The error I get is:
file:///home/username/.guix/sddm-themes/slice/Main.qml: No such file or
directory.

How can I make sddm detect that theme file (Main.qml)?