Remote Theme ineffective

When I use order bundle exec jekyll serve locally, the page seems normal. However, when I push the repo to github, it will be automatically and successfully deployed, and the page looks like this: I noted that #37 has noted that problem in 2021, but I don't get it: what am I supposed to set my base_url? and what's the meaning? Here's part of my configuraion files:

# Gemfile
# frozen_string_literal: true

source "https://rubygems.org"

gem "jekyll-theme-yat"
gem "github-pages", group: :jekyll_plugins
gem 'wdm', '>= 0.1.0' if Gem.win_platform?
group :jekyll_plugins do
  gem "jekyll-feed"
  gem "jekyll-seo-tag"
  gem "jekyll-sitemap"
  gem "jekyll-paginate"
  gem "jekyll-spaceship"

end

# _config.yml
...
remote_theme: "jeffreytse/jekyll-theme-yat"
theme: jekyll-theme-yat

There's 1 warning on Github Action: But I don't know how to fix it.

Hoping to get help, thanks.