fix wrong icon color in app store

[szaimen]

Fix https://github.com/nextcloud/server/issues/42998

For my own testing

docker run -it --rm ^
--name nextcloud-easy-test1 ^
-p 8444:443 ^
-e SERVER_BRANCH=enh/42998/fix-icon-color ^
-e COMPILE_SERVER=1 ^
--volume="nextcloud_easy_test_npm_cache_volume:/var/www/.npm" ^
ghcr.io/szaimen/nextcloud-easy-test:latest