I have never heard of or used flexbox, but a quick google makes it look like it is a CSS tool?
If my findings are correct, then I think that this is the wrong forum for those questions. This forum is related to SQL Server, hence the name "SQL Server Central".
Now, if flexbox is something different than a CSS tool, having more information would likely get you more responses.
If flexbox IS related to CSS, then I recommend looking up tutorials on it such as W3 Schools - https://www.w3schools.com/csS/css3_flexbox.asp
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!
I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.