@Matt, thank you very much for your help!!
But unfortunately I'm still very much lost.
I went into /usr/share/sphinx/themes/basic/layout.html (I think that's the one I have to play with because of something I read about inheritance) but there I only found an empty block named relbar1:
{%- block relbar1 %}{{ relbar() }}{% endblock %}
But I did find a part with a similar name and containing "Navigation":
{%- macro relbar() %}
<div class="related" role="navigation" aria-label="related navigation">
<h3>{{ _('Navigation') }}</h3>
<ul>
{%- for rellink in rellinks %}
<li class="right" {% if loop.first %}style="margin-right: 10px"{% endif %}>
<a href="{{ pathto(rellink[0]) }}" title="{{ rellink[1]|striptags|e }}"
{{ accesskey(rellink[2]) }}>{{ rellink[3] }}</a>
{%- if not loop.first %}{{ reldelim2 }}{% endif %}</li>
{%- endfor %}
{%- block rootrellink %}
<li class="nav-item nav-item-0"><a href="{{ pathto(master_doc) }}">{{ shorttitle|e }}</a>{{ reldelim1 }}</li>
{%- endblock %}
{%- for parent in parents %}
<li class="nav-item nav-item-{{ loop.index }}"><a href="{{ parent.link|e }}" {% if loop.last %}{{ accesskey("U") }}{% endif %}>{{ parent.title }}</a>{{ reldelim1 }}</li>
{%- endfor %}
{%- block relbaritems %} {% endblock %}
</ul>
</div>
{%- endmacro %}
So I decided to try with it. I copied all that part, but only that one, and pasted it into a new file. I saved the file as ~/mySphinxProject/_templates/layout.html (If I understood correctly that is the path and name it must have, please correct me if I am wrong).
On that new file, I replaced the following line
<h3>{{ _('Navigation') }}</h3>
with
<h3>{{ _('xxx') }}</h3>
After running make html, my output disappear, the webpage was completely empty.
I deleted ~/mySphinxProject/_templates/layout.html but I keep having no output at all, the wepage is still completely white.
I might be making more than one mistake... Any help is very much welcome!
By the way, what and where is something named templates_path ?