Can I also influence galleries? For example I want this one being transparent in background instead of black: http://pwdesigns.de/referenzen/referenzen-logos-und-keyvisuals/
Here goes a CSS tip: To change the looks of an HTML element in your page, you need to target it with CSS a selector. A lot of times, a selector will be the id of the HTML element you want to change, or a class that is being applied to it (through the "class" attribute in the HTML). For example, if you take a look at the source code of your page, you will see that the slideshow container is a <div> tag with the class "slideshow-window". Because of that, to remove the background in your slideshow, you can use the following CSS;
This will target <div> tags with the "slideshow-window" class applied to them, remove its background and border. A lot of times, targeting an element through its id or through a class applied to it is enough to get you going.
A good way to figure these things out is to use your browser's developer tools. For example, in Firefox, right click any element you want to change in your page, and select the option "Inspect Element". This will open a panel showing the HTML for the element you clicked, and, on the right, all CSS rules being applied to it. Here is a good post on how to use your browser's developer tools and find CSS selectors: https://dailypost.wordpress.com/2013/07/25/css-selectors/.