If you need inspiration, you can look at the custom BBCodes in the Customisation Database of phpBB.
Here are the steps I took to install and activate the custom style of the board:
After I downloaded it, I copied the SE_Gamer_Dark folder from the archive to the style folder (where the "prosilver" and "all" folders are located). I opened the Administration Control Panel, navigated to the CUSTOMIZE tab and clicked on "Install Styles" under STYLE MANAGEMENT on the left side. Then I ticked the check box on the right side on and confirmed by clicking on the "Install styles" button. After this, I navigated to the "GENERAL" tab and clicked on Board settings under "BOARD CONFIGURATION" on the left side. Under "Board style", I selected the custom style from drop-down lists next to "Default style:" and "Guest style:". I set "Override user style:" to "Yes" and confirmed my changes by clicking on the "Submit" button.