Kosel 1.5.0 question

First of all, it might be wise to upgrade to the latest version of Showkase (v1.6.1) to ensure that any bugs that were present in v1.5.0 and which have since been fixed are not contributing to any layout problems that you may be having.
Please see the Version History for a list of changes between versions.
Instructions for upgrading Showkase can be found here.

This might be quite difficult to achieve. The Kosel theme was designed to have a left-justified site heading and a right-justified navigation menu sharing the same horizontal space.
It might be better (certainly much easier) to use the Boma theme and force Showkase to display the site in small screen mode where the site heading is horizontally centered and the navigation menu is horizontally centered below it.
To see this in action, try viewing this demo Boma themed site in a narrow browser window.
In order to force Showkase to always display Boma pages in small screen mode, you would need to change the breakpoints at which Showkase switches from large to small screen mode.
Open the showkase/_themes/boma/css/styles.css file in a plain text editor, search for instances of:

@media only screen and (min-width: 768px) {

... and change the min-width values to something much larger, such as 9999px (so that the minimum width for large screen mode is never reached).
You might not want to change all the breakpoints, though. You might like to leave the gallery index page breakpoints unchanged to prevent the gallery index page thumbnails from being displayed too large in large browser windows.

Galleries should already be horizontally centered in Showkase pages by default. If you find that they are not, then please post the URLs to your gallery pages so that I can take a look for myself and hopefully help further.
Please note that galleries will not be vertically centered as Showkase pages are designed to scroll vertically rather than display all content in the browser viewport without scroll bars. Juicebox and Showkase galleries should be displayed in their entirety though, without the need for vertical scrolling. If you have a large site heading that you find is pushing your galleries down your Showkase pages, then you can compensate for this by entering a negative value for the 'External fit px' entry in the 'Site -> Customize Theme -> Gallery Pages' section (e.g. '-200').

Add the following CSS to your showkase/_themes/kosel/css/custom.css file:

footer {
    text-align: center;
}

More information on using custom CSS can be found in the Adding Custom Styles support section.

Be sure to re-publish your site after making any changes.

You are free to make whatever CSS modifications you like to your Showkase site but please note that modifications made outside the Showkase interface are not officially supported. (Showkase may expect elements to be in certain positions and will not be aware of any CSS modifications that you may make.)

If you plan to make many custom modifications to a theme, then you might like to consider creating your own theme instead. Please see the Creating Themes support section for details.