How can we support the development of age-appropriate system design in a society with new information and communication technologies (ICT) emerging at an ever-increasing rate, making it imperative to consider age-appropriate design to achieve the best possible usability and acceptance for older adults? This paper describes the development of a style guide within the Historytelling Project to increase the consistency and therefore efficiency of the interface and to gain a higher software development efficiency. The implementation of the style guide is based on the preexisting CSS framework Bootstrap because of its high amount of predefined UI elements. The developed style guide consists of two components: the actual implementation of the style guide in the form of CSS customizations and the associated documentation. Both components are freely accessible and editable online. An adjusted interface was tested against a standard Bootstrap interface with 31 older adults aged 51 to 93 (M = 71), of which 15 participants used the standard Bootstrap interface and 16 the age-adjusted interface. Findings indicate a higher task completion rate (effectiveness) across tasks for the adjusted Bootstrap+ version.