This tutorial explains how to install a XenForo style.
In order to install a style you must have access to forum files and access to forum admin control panel.
Extract zip file containing style.
After extracting zip file, you will find a folder named "styles" that contains all style files. Upload its contents via FTP using an FTP client such as Filezilla to directory "styles" of your forum.
After uploading all files, open your forum in browser, log into admin control panel.
Click "Styles" link and you will see list of installed styles.
Above that list you will see buttons to create new style and import new style. Click "Import a Style" button.
Upload style XML file, click "Import" button.
That will install a new style. But installation is not complete yet. You will probably want to change default style and you will want to be able to customize style.
To update style you will need to import new XML file. Process is same as above, with one minor detail: when importing style click "Overwrite style" option (see screenshot above) and select style you want to update.
Creating child style
You will probably update your style at some point to newer version. Update process requires overwriting old style, which means all custom changes will be lost. How to avoid that? Answer is simple - after installing new style create a child style.
Click "Styles" link and "Create New Style" button.
Select previously installed style as parent style, in "Title" field enter name of your new style. You can enter any name you want.
After new style has been created, you might want to change default style and prevent users from selecting styles they aren't meant to see. To change default style click radio button (rounded box) near your new style. To prevent users from selecting styles they are not supposed to see, disable those styles by unchecking check boxes (square boxes) near those styles.
If after installing new style you do not see images, probably you have ether uploaded style files to wrong directory or style files have wrong permissions. Make sure directory structure is correct by checking part to style.js: it should be styles/stylename/xenforo/style.js
If you have any questions or ran into problems, please post your questions on support forum.