Introduction: While prior research has delved into the emotional aspects of instructional design, it has not extensively examined whether integrating affective features specifically relevant to the theme of the learning materials is essential for enhancing learning effectiveness compared to incorporating general affective features. This study aims to fill this gap by investigating whether the addition of positive features alone can improve a student's learning and whether these positive features have a more significant impact when aligned with and emphasising the theme-relevant content. Methods: Two experiments examined how to improve the instructional effectiveness of a narrated slideshow on lightning formation that employed neutral line-drawings (original group) either by adding colour and smiling faces to key elements (general affective features group) or depicting key elements as colourful cartoon-like characters consistent with the theme of the lesson (theme-specific affective features group). Results: The theme-specific affective features group scored higher on both retention and transfer tests, found the instructor more engaging, and reported feeling happier than the original group (in Experiment 2). The general affective features group rated their instructor as being more engaging than the original group and outscored the original group on both retention and transfer tests in Experiment 2 but not in Experiment 1. Conclusion: Overall, improvement of students' positive emotions and learning outcomes by adding theme-specific affective features is consistent with the Cognitive-Affective Model of Learning with Media and extends previous studies of emotional design. However, adding general affective features unrelated to the lesson theme produced mixed learning outcome results, which require further investigation.