Variable initialization bug in webconfig.py prevents installation of sparse themes

[csams]

A variable initialization bug in webconfig.py prevents installation of themes that don't explicitly set all known colors.

fish, version 3.6.0
Fedora 36 Workstation Edition
xterm-kitty

1. In $HOME/.config/fish/config.fish, set -gx XDG_CONFIG_HOME $HOME/.config/fish
2. Create $HOME/.config/fish/themes
3. Drop a theme into the above directory that doesn't explicitly set all theme related variables
4. Run fish_config and try to set the theme

The follow exception is produced:

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Exception occurred during processing of request from ('::1', 58898, 0, 0)
Traceback (most recent call last):
  File "/usr/lib64/python3.10/socketserver.py", line 683, in process_request_thread
    self.finish_request(request, client_address)
  File "/usr/lib64/python3.10/socketserver.py", line 360, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/lib64/python3.10/http/server.py", line 668, in __init__
    super().__init__(*args, **kwargs)
  File "/usr/lib64/python3.10/socketserver.py", line 747, in __init__
    self.handle()
  File "/usr/lib64/python3.10/http/server.py", line 433, in handle
    self.handle_one_request()
  File "/usr/lib64/python3.10/http/server.py", line 421, in handle_one_request
    method()
  File "/usr/share/fish/tools/web_config/webconfig.py", line 1563, in do_POST
    output += "\n" + self.do_set_color_for_variable(what, "")
UnboundLocalError: local variable 'output' referenced before assignment
----------------------------------------