[evolution] I#1966 - EColorSchemeWatcher: Verify chosen dark variant theme name exists

[Milan Crha <mcrha src gnome org>]

commit 677d5ec7a3ec0d2583e1b048756d88ebdcba953d
Author: Milan Crha <mcrha redhat com>
Date:   Wed Jul 20 18:01:46 2022 +0200

    I#1966 - EColorSchemeWatcher: Verify chosen dark variant theme name exists
    
    Closes https://gitlab.gnome.org/GNOME/evolution/-/issues/1966

 src/e-util/e-color-scheme-watcher.c | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/src/e-util/e-color-scheme-watcher.c b/src/e-util/e-color-scheme-watcher.c
index b5b5f54651..2f49577a8b 100644
--- a/src/e-util/e-color-scheme-watcher.c
+++ b/src/e-util/e-color-scheme-watcher.c
@@ -215,6 +215,13 @@ e_color_scheme_watcher_sync_theme (EColorSchemeWatcher *self)
                        if (self->color_scheme == E_COLOR_SCHEME_PREFER_DARK &&
                            e_color_scheme_watcher_check_theme_exists (theme_name, "dark")) {
                                new_theme_name = g_strconcat (theme_name, "-dark", NULL);
+                               /* Verify whether the newly constructed name can be used; otherwise the theme
+                                  supports the dark variant with the original name. */
+                               if (!e_color_scheme_watcher_check_theme_exists (new_theme_name, NULL)) {
+                                       g_free (new_theme_name);
+                                       new_theme_name = theme_name;
+                                       theme_name = NULL;
+                               }
                        } else if (suffix_cut && e_color_scheme_watcher_check_theme_exists (theme_name, 
NULL)) {
                                new_theme_name = theme_name;
                                theme_name = NULL;