Questions:
Is there a difference beetween a theme and a template for the xoops?
1)If there is, then how can I install the theme?
2)If there is not, then I'l describe what I have done and It didn't work..
I have pressed System Admin->Templates and Uploaded a template (it Was in ZIP, but I uncompressd it and packed to TAR). The theme have apperred in the template manager, but the DEFAULT theme is checked. And there is not any button tj chek this uploaded theme... What I have to do to install it?

Themes and templates are quite different !

A theme is the appearance of your site : colors, fonts, size, images, etc...

Templates are responsable for the layout. The Template Set Manager (System Module -> Templates) allows you to add, delete, edit and duplicate the HTML templates that control block and module layout.

You said you uploaded a template, are you sure it was a template and not a theme? Themes often come in zip files althought templates often come in .gz or .tar files.

To set the default template, go to
System Admin>Preferences>General Settings>Edit

You set the default template there, under Default Template Set

However, what you want to achieve I guess, is changing the theme. For that, you need to unzip the theme zip file and upload the folder of the theme to the 'themes' folder of your XOOPS site. Make sure that the folder you upload actually contains the theme files and not another folder in which the theme files are.

To be sure, once uploaded, you should find something like this :
XOOPS_ROOT/themes/YourNewTheme/some files... (one would be theme.html)

After you've done this, you get back to
System Admin>Preferences>General Settings>Edit

and under Default theme you select YourNewTheme

That's it !

Thanks. It works correctly now.