In admin you download the CSS file in admin > "Tools" > "File manager"> "themes"> "design.css".
After editing the CSS file, upload it again in admin > "Tools" > "File manager" > "themes".
The CSS file can be edited in a regular text editor, e.g. Notes.
Now go to the store to see the changes.
Good knowledge of CSS is required to be able to handle this on your own. We do not provide support for this.
Make a Backup before making any changes by downloading and saving a copy of the CSS file to your computer.
If you delete or overwrite files that you want back, we will charge for the recovery.
Read this if the instructions above are not correct in your store
In older versions, the CSS file is located in "Tools" > "File Manager" > "eigen_css" > "egen.css"
After editing the file, go to admin > "Settings" > "Design, Color & Layout" > "Select color in the store" > "change" and select "own.css".
Then click Update.