Manually replace /opt/cronicle/htdocs/css/style.css with the my style.css
Or if you are on Linux, either run
sudo wget -O /opt/cronicle/htdocs/css/style.css https://raw.githubusercontent.com/iamumairayub/cronicle_dark_theme/refs/heads/main/style.css
or
sudo curl -L -o /opt/cronicle/htdocs/css/style.css https://raw.githubusercontent.com/iamumairayub/cronicle_dark_theme/refs/heads/main/style.css
Then do
cd /opt/cronicle && systemctl stop cronicle && node bin/build.js dist && systemctl start cronicle
You may have to do hard-reload CTRL+R to see changes