It’s late answer for Vipin, but the answer is there in the comments:

git clone https://github.com/nextcloud/client_theming.git
cd client_theming
git submodule update –init
cd client
git submodule update –init
cd ..
sudo apt-get install -y kio-dev
mkdir build-1
cd build-1
cmake -D OEM_THEME_DIR=pwd/../nextcloudtheme ../client
sed -i ‘s/Icon=nextcloud/Icon=Nextcloud/g’ src/gui/nextcloud.desktop
sed -i ‘s/Icon\[\(.*\)\]=nextcloud/Icon\[\1\]=Nextcloud/g’ src/gui/nextcloud.desktop
sudo make
sudo make install