(Here is a screenshot for English version 
)
After you click Pin Topic there is the option to make it a banner 
I’m not sure about changing the behaviour/settings for once a user clicks the x to close it though 
And for editing the welcome topic, I think the welcome topic is one of the first topics made by system by default, so you can go into system’s topics to find it again if you want to edit that 
but I’m not sure if that’s what you wanted