Thanks for your trust Damo 
First things first, Grav is not like Joomla in regards to theming. I suggest you quickly read this blog post as I already explained there some key differences.
So after reading that you understand that each theme in Grav can be much more unique than in Joomla/WordPress. There are no ‘core’ features it must support, so most of the stuff you see in a particular theme, is actually delivered by that theme, not Grav itself. Moving ‘functionality’ usually means copying content + twig template files.
That said, the best way to get a site up and running with a new theme is to start with that theme via it’s Skeleton package (think RocketLauncher) if it has one. This is because the skeleton contains content + theme + plugins + config, all ready to go. You can add the admin plugin to the skeleton if you wish with:
bin/gpm install admin
---
From the CLI (We will start adding the admin as part of skeletons soon too).
Ok, that said, your questions are specifically about mo difying the TwentyFifteen theme and/or the config for that theme. You might want to consider inheriting the theme and making your modifications, so any update to the theme itself down the line is not lost. [See the docs for this](http://learn.getgrav.org/themes/customization#theme-inheritance).
Regarding your questions:
1) This will be solved if you actually start with the Skeleton, as it's a bit involved in explaining how to do this to a newbie, no offense :), Grav is just much more controlled by the theme (as explained in the blog post above) than you thinking. - https://github.com/getgrav/grav-skeleton-twentyfifteen-site/releases/download/1.0.3/grav-skeleton-twentyfifteen-site-v1.0.3.zip - [demo](http://demo.getgrav.org/twentyfifteen-skeleton)
2) To modify the title, i'm not 100% familiar with this theme, so i poked about. The sidebar is rendered by `templates/partials/sidebar.html.twig`, and in there I see this code:
<header id="masthead" class="site-header" role="banner">
<div class="site-branding">
<h1 class="site-title"><a href="{{ base_url_absolute }}">{{ 'SITE.TITLE'|t }}</a></h1>
<p class="site-description">{{ 'SITE.DESCRIPTION'|t }}</p>
<button class="secondary-toggle">Menu and widgets</button>
</div>
</header>
This tells me that the title is set by `SITE.TITLE|t` which means its a language string as it has the translate filter (`|t`) applied. So I looked in `languages.yaml` and i see this:
en:
SITE:
TITLE: My Blog
DESCRIPTION: Blogging about stuff
So that's where you change it. It will change depending on language if you have multiple languages supported, else it defaults to english.
