Google says: It's a Marbled Reed Frog (Hyperolius marmoratus) ... but I'm not the expert.

I got the following answer from an expert outside RG:

"It’s a member of the Hyperolius viridiflavus complex – probably H. glandicolor. That’s the natural colour! Abundant species."

I suggest to check the book "Treefrogs of Africa" by Arne Schiotz (1999). From my long-term experience on Lepidoptera and plants in central and coastal Kenya I doubt that this might be H. glandicolor since it occurs, based on the very good book mentioned above, only in the area of the Taita Hills. The latter form the northermost part of the Eastern Arc Mountains, resemble the Kenyan Highlands (including the Ngong Forest and Karen) in ecoclimatic parameters but were found to be distinct from either. Additionally, H. glandicolor of the viridiflavus subgroup (representing often small forms of less than 35mm) has a dark dorsum, almost black with white spots and streaks, but your picture shows a very different species that might also belong to another and larger genus e.g., Chiromantis.