vcl: Use ImplImageTree member instead of static variable

Introduce new ImplImageTree::mbPreferDark member to
replace the static variable introduced in

    commit 3a9ee3a2a7
    Date:   Sat Sep 26 12:14:17 2026 +0200

        Resolve tdf#173739 - Consider dark UI for icon fallback

, see also discussion in [1].

There's only a single ImageTree (and therefore) ImplImageTree
instance that can be retrieved using ImageTree::get.

While at it, also switch the formatting/indentation of the
`else if` in ImplImageTree::fallbackStyle to a more conventional
one.

[1] https://gerrit.libreoffice.org/c/core/+/211478/comments/d66b922c_086d381c

Change-Id: Ib5cbcd5ae0e227b4eecad256d69cbd07579d8c73
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/211553
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/inc/implimagetree.hxx

@@ -129,6 +129,8 @@ private:
     /// Style used for the current operations; switches switch several times during fallback search.
     OUString maCurrentStyle;
 
+    bool mbPreferDark;
+
     IconSet& getCurrentIconSet()
     {
         return maIconSets[maCurrentStyle];
@@ -162,7 +164,7 @@ private:
 
         Must not be cyclic :-)  The last theme in the chain returns an empty string.
     */
-    static OUString fallbackStyle(std::u16string_view rStyle);
+    OUString fallbackStyle(std::u16string_view rStyle);
 };
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */

vcl/source/image/ImplImageTree.cxx

@@ -63,8 +63,6 @@
 
 using namespace css;
 
-static bool bPreferDark; //initialized in ImplImageTree c'tor
-
 bool ImageRequestParameters::convertToDarkTheme()
 {
     static bool bIconsForDarkTheme = !!getenv("VCL_ICONS_FOR_DARK_THEME");
@@ -225,7 +223,7 @@ void loadImageFromStream(std::shared_ptr<SvStream> const & xStream, OUString con
 
 ImplImageTree::ImplImageTree()
 {
-    bPreferDark = ThemeColors::GetThemeColors().GetWindowColor().IsDark();
+    mbPreferDark = ThemeColors::GetThemeColors().GetWindowColor().IsDark();
 }
 
 ImplImageTree::~ImplImageTree()
@@ -353,11 +351,10 @@ OUString ImplImageTree::fallbackStyle(std::u16string_view rsStyle)
         sResult = "breeze_dark";
     else if (rsStyle == u"colibre_dark" )
         sResult = "colibre";
+    else if (mbPreferDark)
+        sResult = "colibre_dark";
     else
-        if (bPreferDark)
-            sResult = "colibre_dark";
-        else
-            sResult = "colibre";
+        sResult = "colibre";
 
     return sResult;
 }