When i run leafpad, it generates a whole bunch of Gtk warnings as
shown below [1]. I tracked this down to my XFCE theme Greybird.
/usr/share/themes/Greybird/gtk-2.0/gtkrc, which depends on the
murrine engine. There is a slackbuild for it [2], but i thought it
was odd that Slackware 15.0 would include a theme without also including
its dependencies. I switched back to the Adwaita theme and that
cleared up the Gtk warnings. This is related to the thread below [3].

[1]
bash-5.1$ leafpad

(leafpad:31963): Gtk-WARNING **: 09:07:11.581: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.582: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.582: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.582: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.582: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.582: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.582: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.582: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.582: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.583: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.583: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.583: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.583: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.583: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.583: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.583: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.583: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.584: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.584: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.584: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.584: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.584: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.584: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.584: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.584: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.584: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.584: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.585: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.585: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.585: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.585: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.585: Unable to locate theme engine in module_path: "murrine",

(leafpad:31963): Gtk-WARNING **: 09:07:11.585: Unable to locate theme engine in module_path: "murrine",

[2]
https://slackbuilds.org/result/?search=murrine&sv=15.0

[3]
https://www.linuxquestions.org/quest...em-4175463210/

Got that on 15.0, it's one of those GTK+2 engines which hasn't been ported to GTK+3
Thought about merging it with gtk-engines package locally, but decided to leave it like that.
Your issue is with greybird package providing GTK+2 theme without the required engine.

The issue with the missing "murrine" theme engine was also discussed in the "Requests for -current (14.2 --> 15.0)" thread. The relevant posts are listed below.

https://www.linuxquestions.org/quest...ml#post6246159
https://www.linuxquestions.org/quest...ml#post6246170
https://www.linuxquestions.org/quest...ml#post6249885
https://www.linuxquestions.org/quest...ml#post6249907
https://www.linuxquestions.org/quest...ml#post6250134

While it may seem odd that a theme (Greybird) included in Slackware does not include the engine required by that theme, in truth this only affects the Greybird theme when gtk2 is in use. Greybird with gtk3 programs is fine because it doesn't use the murrine engine in this case. The declining use of gtk2 and the orphaning of the murrine engine make it unlikely that murrine will be included in Slackware. As explained in post 6249885, it wasn't difficult to get the murrine engine running under what was then Slackware64-current and there is also an SBo for it. I don't expect the compilation to be any harder under Slackware64 15.0.

In the end, the lack of the murrine theme engine doesn't stop gtk2 programs from running. It only causes cosmetic effects: the messages reported by the OP and a lack of theming for gtk2 programs when Greybird is selected as the theme.