The discussion revolves around adding custom HTML and CSS to a Discourse forum. waqar initially reports an issue with not being able to add custom HTML and CSS, having mistakenly removed the default theme that allowed this option.
Darkpixlz identifies the problem, pointing out that the issue lies with the theme being used, and suggests creating a custom theme or forking a GitHub repository to edit it. waqar asks for help in identifying the original theme that allowed custom HTML and CSS, but Darkpixlz advises that only self-made themes or forked repositories can be edited.
waqar discovers that custom themes can be created without coding, and Darkpixlz provides instructions on how to do so. Jagster agrees with Darkpixlz, suggesting that creating a custom theme or using components is the best approach, rather than forking from GitHub, and raises a point about the trade-offs between customizing themes and losing updates.
That’s your issue right htere.
Also, you cant edit themes you get fom GitHub. If you have to edit it that bad, then take the code from the GitHub repository and paste it in yourself.
thank you for reply. Which theme I can edit? Firstly there was some theme I was able to add custom html/css to that theme but I don’t remember the name. Do you know which one was that and how I can activate that again?
You can edit the themes that you make yourself. You can also fork the GitHub repositories and edit them there 
Just click “Create new theme”, then go wild on the CSS and HTML.
You can also have components which add on to themes.
This is the right way 
Not making copy/fork from GitHub.
And as usual there is but. For me anyway — if WordPress world is familiar the question is very much same as there should one use functions.php for theme dependent changes, something like code snippets when change is not theme dependent or child theme if must edit theme or plugin; and if child theme is in use one will loose updates of that part.
