I just made a child theme so nothing will be lost on an update, correct? ??
Only the changes you transfer into the Child Theme will be saved.
The thing is, I would like to add a few new template pages with an other css structure.
So then I’ll have to go as you explained?
Yes, for each page.
Will your line overwrite the available css?
If you’re going to create new pages and assign the template files to those pages, those new pages will have different page IDs.
Page ids are available within the <body>
HTML element, when the page loads. You can see what these are by using a Browser Inspector Tool like FireBug.
Then you can pick these IDs off and use them for CSS.
The CSS code I exampled is using the page ID, so it will only apply styles to that page and not override pervious styles.
Other issue, on the homepage I would like to introduce a slideshow in the header.
These sorts of things can be achieved with slideshow plugins, but I wouldn’t know which is best to recommend. There are many.
Last item… On all pages I want to change the navigation, style & function. Is this a nasty work to do?
Sorry, I don’t understand what specifically you want to do. Apply a style to the last menu item?