Add a qt.force_platformtheme setting

doc/changelog.asciidoc

@@ -31,6 +31,8 @@ Added
 - New `content.site_specific_quirks` setting which enables workarounds for
   websites with broken user agent parsing (enabled by default, see the "Fixed"
   section for fixed websites).
+- New `qt.force_platformtheme` setting to force Qt to use a given platform
+  theme.
 
 Changed
 ~~~~~~~

doc/help/settings.asciidoc

@@ -245,6 +245,7 @@
 |<<prompt.radius,prompt.radius>>|Rounding radius (in pixels) for the edges of prompts.
 |<<qt.args,qt.args>>|Additional arguments to pass to Qt, without leading `--`.
 |<<qt.force_platform,qt.force_platform>>|Force a Qt platform to use.
+|<<qt.force_platformtheme,qt.force_platformtheme>>|Force a Qt platformtheme to use.
 |<<qt.force_software_rendering,qt.force_software_rendering>>|Force software rendering for QtWebEngine.
 |<<qt.highdpi,qt.highdpi>>|Turn on Qt HighDPI scaling.
 |<<qt.low_end_device_mode,qt.low_end_device_mode>>|When to use Chromium's low-end device mode.
@@ -3036,6 +3037,16 @@ Type: <<types,String>>
 
 Default: empty
 
+[[qt.force_platformtheme]]
+=== qt.force_platformtheme
+Force a Qt platformtheme to use.
+This sets the `QT_QPA_PLATFORMTHEME` environment variable which controls dialogs like the filepicker. By default, Qt determines the platform theme based on the desktop environment.
+This setting requires a restart.
+
+Type: <<types,String>>
+
+Default: empty
+
 [[qt.force_software_rendering]]
 === qt.force_software_rendering
 Force software rendering for QtWebEngine.

qutebrowser/config/configdata.yml

@@ -178,6 +178,19 @@ qt.force_platform:
     This sets the `QT_QPA_PLATFORM` environment variable and is useful to force
     using the XCB plugin when running QtWebEngine on Wayland.
 
+qt.force_platformtheme:
+  type:
+    name: String
+    none_ok: true
+  default: null
+  restart: true
+  desc: >-
+    Force a Qt platformtheme to use.
+
+    This sets the `QT_QPA_PLATFORMTHEME` environment variable which controls
+    dialogs like the filepicker. By default, Qt determines the platform theme
+    based on the desktop environment.
+
 qt.process_model:
   type:
     name: String

qutebrowser/config/configinit.py

@@ -106,6 +106,8 @@ def _init_envvars() -> None:
 
     if config.val.qt.force_platform is not None:
         os.environ['QT_QPA_PLATFORM'] = config.val.qt.force_platform
+    if config.val.qt.force_platformtheme is not None:
+        os.environ['QT_QPA_PLATFORMTHEME'] = config.val.qt.force_platformtheme
 
     if config.val.window.hide_decoration:
         os.environ['QT_WAYLAND_DISABLE_WINDOWDECORATION'] = '1'

tests/unit/config/test_configinit.py

@@ -308,6 +308,7 @@ class TestEarlyInit:
         ('qt.force_software_rendering', 'chromium',
          'QT_WEBENGINE_DISABLE_NOUVEAU_WORKAROUND', '1'),
         ('qt.force_platform', 'toaster', 'QT_QPA_PLATFORM', 'toaster'),
+        ('qt.force_platformtheme', 'lxde', 'QT_QPA_PLATFORMTHEME', 'lxde'),
         ('window.hide_decoration', True,
          'QT_WAYLAND_DISABLE_WINDOWDECORATION', '1')
     ])