Oh wait! So it's okay to just use f2u chat widget i found somewhere on the internet and just change it a bit to suit my needs? People do that? I am just used to art community rules where it's bad to use other person's work, like tracing and etc
It really depends!
For software, there are different licenses in use - some of them put rather harsh restrictions, other allow modification under certain circumstances (very often, shipping the source codes, linking the original, and allowing others to do the same - this licenses are often called free/libre/open source), few have no restrictions in regard of use or modification at all.
The Slime2 thing that I linked has following terms of use:
Do not claim this as your own or sell it
You may modify the simple-chat widget to create your own slime2 widget, and you can freely share or sell your widget
Feel free to post your widget here on the forums: https://forums.slime2.stream/resources/categories/themes.2/
if you put a link to the one you have in mind, I can look if I can tell you more about it. If there is no license or information given along (and you want to be secure), you might have to ask the creator for approval.