Add the following code to the '_themes/kosel/css/custom.css' file:

Showkase creates thumbnails for Juicebox-Pro galleries at the dimensions specified by the thumbWidth and thumbHeight configuration values in the interface. If you change these values after creating a gallery, the thumbnails will not be displayed at their actual size in the gallery and they may look blurred. To recreate the thumbnails at the new dimensions, edit the gallery page, click on the 'Advanced' tab and select 'Rebuild this Juicebox gallery'.

This looks like a bug. Thank you for reporting. I have logged a bug report with the developers and it should be fixed in the next version of Showkase. In the meantime, open the 'admin/plugins/juicebox/defaults.ini' file in a plain text editor, search for the text 'BackButtonUseIcon' and change it to 'backButtonUseIcon' (with a lowercase 'b' at the start).

The watermarking functionality is a feature found only in JuiceboxBuilder-Pro and is not available in Showkase.
If you like, you could create a gallery with JuiceboxBuilder-Pro (using a watermark), upload the gallery folder to your web server and import the gallery into your Showkase installation in the 'Site -> Import' section.

you would need to 'Rebuild' your gallery ('Edit Gallery Page -> Advanced Tab -> Rebuild') after making the change.
(I tested this fix myself and it works.)

The entries in the 'defaults.ini' file should not reflect your own gallery's settings. (This contents of this file will not change.) They are default values used to determine whether or not it is necessary to include a configuration option in a gallery's 'config.xml' file. (If a custom settings in a gallery matches the configuration option's default value, then Showkase knows that there is no need to enter the setting in the gallery's XML file.)

There is no way to watermark images in a Juicebox gallery created within Showkase.
You can enter a relative or absolute URL in the 'Watermark Url' text field for SimpleViewer-Pro galleries, though.
This is because the watermarking in SimpleViewer-Pro superimposes the watermark image on top of the main image at the time the gallery is displayed. The watermarking in JuiceboxBuilder-Pro embeds the watermark into the main images themselves when the images are resized at the time the gallery is created (not when it is displayed).

Check that you do not have any blank lines in the body text of your gallery page. (Edit the gallery page, click in the text editor area and ensure that everything within the box has been deleted.)
If this does not help, please post the URL to your web page so that I can take a look. It would help to see the web page in question to know whether the space you are referring to is within the gallery itself (a Juicebox issue) or outside the gallery (a Showkase issue).

I have notified the developers of this and it should be fixed in the next version of Showkase. In the meantime, add the following code to the '_themes/kosel/css/custom.css' file:

a.jb-go-back-icon {
    border: none;
}

Thank you for providing the URL to your web site. The gap you mention is the area for the body text (which is your case, is empty). With nothing entered for the 'Page title' and in the text editor, the gap you see on your pages is the minimum height of the body text area.
If you simply do not want to display the body text area at all (removing the gap completely), add the following code to the '_themes/kosel/css/custom.css' file:

section.page-body {
    display: none;
}

Unfortunately all content of welcome and about pages disappeared and the upper half of contact too:(

If you want to remove the body text area from Juicebox gallery pages only, then use the following code in the '_themes/kosel/css/custom.css' file:

body.type-juicebox section.page-body {
    display: none;
}

The gap beneath the thumbnails on the Gallery Index Page is to accommodate long gallery titles (which can wrap onto three separate lines before truncating). If you want only one line of text to be visible beneath each gallery thumbnail, try using the following code in the '_themes/kosel/css/custom.css' file. However, please note that I do not know what knock-on effects (if any) this may have.

body.type-galleryindex .content figcaption {
    height: 1.25em;
}

Each page has its own separate layout and the spacing on each page may be due to different elements (with different CSS classes).
If you want to customize your Showkase pages beyond what can be achieved in the interface, then I would recommend using your browser's developer tools to determine the element that you wish to change and then add custom CSS to the theme's 'custom.css' file.