Cleanup: gchar as gchar instead of char

Use gchar instead of char in most of the cases where gchar is intended.

Reason: improve compatibility and stability. Issue #1819

Minor refactoring.

src/config/theme.c

@@ -686,7 +686,7 @@ _theme_prep_fgnd(char* setting, GString* lookup_str, gboolean* bold)
 char*
 theme_get_string(char* str)
 {
-    char* res = g_key_file_get_string(theme, "colours", str, NULL);
+    gchar* res = g_key_file_get_string(theme, "colours", str, NULL);
     if (!res) {
         return strdup(g_hash_table_lookup(defaults, str));
     } else {