Hi Thórralf
and welcome in Neos-Universe.
I will start upside down:
1. Hints to other threats or howto's are welcome:
My recommendations: https://discuss.neos.io/t/layover-text-background-image/1194/3
But unfortunately they are german how-to's. Maybe you can Google persuade to help you with translation ... The ebook-neos-template.pdf is really a good startpoint to understand the basics/principles of NEOS.
I guess, you know the official doku: http://neos.readthedocs.io/en/latest/
But for my first steps, was too arid and abstract (But I'm not english nativ – so for other, will be more easy)
On the other side, write doku is a huge effort of work and time. All in all I'm more than happy with – in combination with the great discuss.neos.io-forum.
2.Why these elements are not as simple to edit like the contents?
To be honest, the deeper reason I don't know, because I only make websites with Neos with pleasure. So I only can guess and give you a hint, how you can change:
a: my guess: Members of core-team are decision-maker and most of them come from web-agencies. Of course they search improvement in their everyday work in first priority. And I’m very happy with!
One-click solutions (change logo with one click «like content») in template/layout-parts are not their ever-day work, nor persuasion. Strict separation from content and template is almost like a religion 
. Maybe logo is more template-part then content?
b: hint: (maybe is better to do research for basics, before you read my semi-detailed explanations)
- In Neos you will find always an main-typoscript – uperordinate, as the backbone of the whole website. path: «Sites/SiteName/Resources/Private/TypoScript/Root.ts2».
(New name for Typoscript [to close to typo3 with other tasks] is «Fusion» but for understand howto’s and doku: both mean same)
- therein you find all the main-stuff and -logic for the website. Also the body-part:
Therein are the directives to render the template «Default.html» for the hole Website and«MainMenu.html» for the Menu-Part.
(Templates in Neos-Universe: «Fluid».)
In this Fluid-Template (path:Packages/Sites/SiteName/Resources/Private/Templates/TypoScriptObjects/MainMenu.html) you will find the section «itemList».
therein is the logo defined as svg-element:
if you change this to something else:
your demo.site will change:
BUT: Don't do so!
To be honest, in my opinion Neos is much more integrator-, editor- and developer-friendly then every other cms I know. Also flexible with almost no limit, but it is also more complex to find in, then in Joomla, Wordpress, Contao, ...
In this CMS it is no good idea to do stuff with «try & error» in Fluid-templates, Fusion-scripts, EEL, FlowQuery and so, before you have at least a bit better overview.
**There are to many things where you could stumbling over without noticing why. And you will loose time and pleasure!**
To loose pleasure would be a shame, because Neos is to genius to put it too early aside.
Better start from scratch with simple own templates, own ContentTypes, small fusion (typoscript), flowQuery directives, and so on, to understand the parts for its own.
«Step by step» is really worth in Neos-Universe. (This is my own lesson from some very harsh steps.)
Demo-site: (I guess you are talking from abot the logo)
In my opinion, the demo-site is not a good starting-point for new websites. It is a working-reference to recognize what you can do with Neos. Also to study the code, and lern how you could find the way to a solution, and at the end: find your own way. Because in Neos there are tons of ways to do something. That means «flexible with almost no limit».
Changing the CSS like font etc...
Create and include your own Fluid-Template with your own CSS.
Hope my answer will help more than confuse you
Have fun with Neos
Cheers
