The Customize Stage (ステージカスタム) screen allows you to customize your stage's speakers and background. Clicking the Livehouse from your Town also acts as a shortcut to this page.

Translated Stage Customization screen
You can tap the buttons to change the selected equipment after purchasing the objects from the Stage Equipment Shop or Value Packs Shop, or from earning them through a campaign. Select by Set (セットで選択) changes all the features on your stage at the same time, given that you own all the equipment in the set. After making any equipment changes, the Confirm (決定) button at the bottom right will save the changes.
The more Equipment that you own, the higher the score boosting effect the stage will give. Each object adds a +0.5% stat bonus and there is a maximum bonus of +100%. The equipment does not need to be placed on the stage in order to provide the bonus. Your current bonus can be seen at the top right of the screen where it says Stage Effects (ステージ効果). The maximum stage effects bonus you can currently have is +56%.