Create a new theme

To create a new theme, create a new theme directory (mkdir ~otrs/Kernel/Output/HTML/NewTheme/). Copy an existing theme into the new directors (cp ~otrs/Kernel/Output/HTML/Standart/*.dtl ~otrs/Kernel/Output/HTML/NewTheme/).

Modify the dtl files like you want.

Add a new database entry for this theme. You have to do this via SQL.
shell:~> mysql -u root -p some-pass otrs
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 1 to server version: 3.23.48-log

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> INSERT INTO theme
    ->     (theme, valid_id, create_by, change_by, change_time)
    ->     VALUES
    ->     ('NewTheme', 1, 1, 1, current_timestamp);
mysql>

Ready. Goto the preferences page and change your theme.