I can't see either of your screenshots, but normally you can change the windowskin at line 420:
# WINDOWSKIN - The windowskin for each window in the Quest scene. It must
# refer to a graphic in the System folder of Graphics. If set to false, then
# it will use whatever windowskin is default. If you are using a script which
# lets the player choose the windowskin, false is the recommended value.
WINDOWSKIN = false
Instead of false, put:
WINDOWSKIN = "Window"
Replace "Window" with the name of whatever your windowskin is, but leave the quotation marks.
As for the disappearing text, I am not sure why that would be happening. My best guess is that whatever windowskin you are using has no colour palette. I.e. a typical windowskin looks like this:
(https://rmrk.net/index.php?action=dlattach;topic=50035.0;attach=29737)
The 32 blocks in the bottom-right hand corner govern the colour of text. If those are missing, then text would not be drawn.
Just so you know though, you don't have to change the windowskin if all you want to do is make it invisible. Around line 428 you should see this:
# WINDOW_OPACITY - The opacity of the windows in the Quest scene. If set to
# false, it will use the default opacity for windows.
WINDOW_OPACITY = false
Just change it to:
WINDOW_OPACITY = 0
You can use any value between 0 and 255, with 0 being fully transparent and 255 being fully opaque.
Well, did you edit the script directly before I responded? I noticed that in the topic, boe had suggested directly modifying code in the non-editable region of the script. His advice wasn't wrong, but sepending on where you inserted that code that he suggested, it may be interfering with the ordinary operation of the script.
Try retrieving a fresh copy of the script (but preserve whatever quests you have made).
Also, instead of deleting the .activate at the part where you did, just replace that line with:
def on_category_ok; @quest_list_window.activate if @quest_list_window; end