Hello Darek,
This is a long request. It would have been easier if you have break down your list in different thread.
1. How can I change the font size for the H1 or H2 tags?
— I have seen that you have already change the font size of heading of page title. For page title the css will be ad below:
.entry-title { font-size: 18px; }
For normal h1 and h2 you can use the following:
h1 { font-size: 34px; }
h2 { font-size: 22px; }
2. How can I change the font style for the H2 tag and for the menu nav bar?
— For menu bar font size change.
#access ul li a { font-size: 13px; }
3. How can I change width for the menus in nav bar? I want to move “Kontakt” menu to the right or to stretch all menus a bit to make it look good.
— For this please “Custom Menu” features. See this http://en.support.wordpress.com/menus/
To stretch them menu you can adjust the padding as needed
#access ul li a { padding: 0 18px; }
4. How to change the height of post image and text box on main page?
— It is not recommended to change the height of the post image in the main page. For text box you can define the excerpt length through “Appearance -> Theme Options -> Theme Settings -> Excerpt / More Tag Settings. Increase the Excerpt Length.
5. How I can turn off “comment’s box” for “Kontakt” page or other ?
— You can turn off from though your “Discussion Box” in your page. See this http://en.support.wordpress.com/enable-disable-comments/
6. How I can change background in comment’s ?
— For post author background color
.commentlist > li.bypostauthor { background-color: #DDDDDD; }
— For others background color
.commentlist > li.comment { background-color: #DDDDDD; }
7. How I can change the font size and style in “Leave a comment” and “1 comment, 2 comments, etc” above comments box for example here : http://www.poradnikniemowlaka.pl/?p=86
— For font size
#comments-title, #reply-title { font-size: 25px; }
For more please see the css tutorial http://www.w3schools.com/css/
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