Version-independent jstree: fix references

72ce416 changed /assets/jstree-VER
to /assets/jstree.  This commit changes the references to that directory
to refer to the new one.

Also, a falsy custom-scrollbar color is not an error; it's the same as
no custom-scrollbar color being specified.

app/settings/manifest.js

@@ -514,7 +514,7 @@ recovered window will show up in TabFern as a separate, unsaved window
     listed in the title bar.  This will make it easier to reference the
     commit in bug reports.  ${issue(210)}</li>
 <li>Fix broken image links in README.md.  Thanks, @bluikko!  ${issue(224)}</li>
-<li>Misc: ${issue([233, 236, 237], true)}</li>
+<li>Misc: ${issue([233, 236, 237], true)}; parts of ${issue(240, true)}.</li>
 ` +
 '</ul>'
             ),

app/win/main_tl.js

@@ -4004,6 +4004,11 @@ function preLoadInit()
     //Custom skinny-scrollbar color
     CSSC: if(S.getBool(S.SKINNY_SCROLLBARS) && document.styleSheets) {
         let color = S.getString(S.S_SCROLLBAR_COLOR);
+        if(!color) {
+            log.info("No custom color for skinny scrollbars");
+            break CSSC;
+        }
+
         color = Modules.tinycolor(color);
         if(!color.isValid()) {
             log.error({'Invalid custom color for skinny scrollbars':
@@ -4021,10 +4026,10 @@ function preLoadInit()
             // override works and we don't have to remove the existing rule.
             // Note: color.toRgbString() for formatting consistency.
         }
-    }
+    } //CSSC
 
     let url = chrome.runtime.getURL(
-                `/assets/jstree-3.3.4/themes/${S.getThemeName()}/style.css`);
+                `/assets/jstree/themes/${S.getThemeName()}/style.css`);
     let before = document.getElementById('last-stylesheet');
     loadCSS(document, url, before);
 

static/t/index.html

@@ -12,7 +12,7 @@
   <link rel="stylesheet" href="test.css">
 
   <!-- stylesheets for jstree testing -->
-  <link rel="stylesheet" href="/assets/jstree-3.3.4/themes/default-dark/style.css">
+  <link rel="stylesheet" href="/assets/jstree/themes/default-dark/style.css">
 
 </head>