Describe the solution you'd like.
I think the Dragonflight close button art doesnt really fit the AddOn and I would like to see the pre 10.x close button added.
I am a AddOn developer myself, and its very simple to achieve this, if you're interested. all you have to do is:
in frame.lua under
f.CloseButton:SetPoint('TOPRIGHT', -2, -2)
add:
f.CloseButton:SetSize(32, 32)
f.CloseButton:SetDisabledTexture("Interface\Buttons\UI-Panel-MinimizeButton-Disabled")
f.CloseButton:SetNormalTexture("Interface\Buttons\UI-Panel-MinimizeButton-Up")
f.CloseButton:SetPushedTexture("Interface\Buttons\UI-Panel-MinimizeButton-Down")
f.CloseButton:SetHighlightTexture("Interface\Buttons\UI-Panel-MinimizeButton-Highlight")

Any additional context?
Screenshot of the closebutton with the code above