From 66ba9127acfa27e732c57587e2c12e8d4e8ed8a7 Mon Sep 17 00:00:00 2001
From: Hombre57 <natureh.510@gmail.com>
Date: Sun, 20 Aug 2017 00:57:30 +0200
Subject: [PATCH] Solving issue #3998

---
 rtgui/main.cc | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/rtgui/main.cc b/rtgui/main.cc
index 455749fdd..731de81cd 100644
--- a/rtgui/main.cc
+++ b/rtgui/main.cc
@@ -261,8 +261,30 @@ RTWindow *create_rt_window()
 #endif
 
         Glib::RefPtr<Glib::Regex> regex = Glib::Regex::create(THEMEREGEXSTR, Glib::RegexCompileFlags::REGEX_CASELESS);
-        Glib::ustring filename = Glib::build_filename(argv0, "themes", options.theme + ".css");
-        if (!regex->match(options.theme + ".css") || !Glib::file_test(filename, Glib::FILE_TEST_EXISTS)) {
+        Glib::ustring filename;
+        Glib::MatchInfo mInfo;
+        bool match = regex->match(options.theme + ".css", mInfo);
+        if (match) {
+            // save old theme (name + version)
+            Glib::ustring initialTheme(options.theme);
+
+            // update version
+            auto pos = options.theme.find("-GTK3-");
+            Glib::ustring themeRootName(options.theme.substr(0, pos));
+            if (GTK_MINOR_VERSION < 20) {
+                options.theme = themeRootName + "-GTK3-_19";
+            } else {
+                options.theme = themeRootName + "-GTK3-20_";
+            }
+            // check if this version exist
+            if (!Glib::file_test(Glib::build_filename(argv0, "themes", options.theme + ".css"), Glib::FILE_TEST_EXISTS)) {
+                // set back old theme version if the actual one doesn't exist yet
+                options.theme = initialTheme;
+            }
+        }
+        filename = Glib::build_filename(argv0, "themes", options.theme + ".css");
+
+        if (!match || !Glib::file_test(filename, Glib::FILE_TEST_EXISTS)) {
             options.theme = "RawTherapee-GTK";
             // We're not testing GTK_MAJOR_VERSION == 3 here, since this branch requires Gtk3 only
             if (GTK_MINOR_VERSION < 20) {