shell: set QT_QPA_PLATFORMTHEME env

Hopefully fixes most people's icon issues

Fixes caelestia-dots/shell@390

src/caelestia/subcommands/shell.py

@@ -1,3 +1,4 @@
+import os
 import subprocess
 from argparse import Namespace
 
@@ -32,7 +33,12 @@ class Command:
                 args.append("-d")
                 subprocess.run(args)
             else:
-                shell = subprocess.Popen(args, stdout=subprocess.PIPE, universal_newlines=True)
+                shell = subprocess.Popen(
+                    args,
+                    stdout=subprocess.PIPE,
+                    universal_newlines=True,
+                    env=dict(os.environ, QT_QPA_PLATFORMTHEME="gtk3"),
+                )
                 for line in shell.stdout:
                     if self.filter_log(line):
                         print(line, end="")