How to render a custom template in pre-existing theme

Sorry for asking something again @pamtbaau @pmoreno … but is there specific documentation/rules regarding creating a custom template within an original theme and its linking to the custom.css? I’ve been hours troubleshooting why the custom template does not link to the custom.css file.
I have tried the following AND each time I have tried each step listed below I also done both of the below:
a) Have cleared cache like you showed me above (bin/grav cache)
b) in system.yaml I have tried with ‘assets: css_pipeline: false’ and with ‘assets: css_pipeline: true’ (this one because I saw your answer @pamtbaau here: Css custom attributes not working)
For context: the body of the HTML is marked all with a TWIG {% block %}. Like: {% block content %} <‘body’> … </‘body’> {% endblock %}

{% block stylesheets %}
{% do assets.css(‘theme://css/custom.css’, 100) %}
{% endblock %}
→ did not work. I also tried without integer and with the following integers: 101, 105, 110 and 10.

<'link type=“text/css” rel=“stylesheet” {{ source(‘custom.css’) }}">
→ I found this online and I tried… It did not work, I get the mistake of "Template “…/assets/css/custom.css” is not defined & it points me to the ‘grav-admin/system/src/Grav/Common/Twig/Twig.php’ and there I don’t see what should I touch, therefore I prefer to not mess up.

{% block stylesheets %}
{% do assets.addCss(‘theme://css/custom.css’, 101)|raw %}
{% endblock %}
→ Since the HTML is still in pure without the TWIG, I thought that maybe adding raw would help (after reading so much Q&A online I just start trying things…)

{% block stylesheets %}
{% do assets.addInlineCss(‘theme://css/custom.css’) %}
{% endblock %}
→ Thought this could be an option to make it work but also not.

{% extends ‘@future2021/partials/base.html.twig’ %}

{% block stylesheets %}
{{ parent() }}
{% do assets.addCss(‘theme://css/custom.css’) %}
{% endblock %}
→ I thought that maybe by linking it to another template, the custom one would render something that I am currently missing (by ‘something’ I imagine some type of connection with the asset manager)

{% block stylesheets %}
{% do assets.addCss(‘theme://future2021/assets/css/custom.css’) %}
{% endblock %}
→ I started doubting the syntax of the link, maybe I am just not linking it correctly. I have tried several paths, none with positive result.

I have found information about customizing a full theme using inheritance, but I am not able to find an explanation for such a basic problem of linking the css. Is this something I am trying (have a totally custom Homepage to a original theme) something too out of the box?
Asking because if it’s something GRAV is not thought for, then I stop trying, because I clearly don’t know enough to be making it happen and I don’t want to bother more with rookie questions.

Thanks a lot for any help