update light/dark codeblock styles for visibility

...s/Admin/ExperimentalFeatures/Features/LiveSync/toggle.jsx

frontend/src/pages/FineTuning/Steps/DataUpload/index.jsx

...Settings/EmbedConfigs/EmbedRow/CodeSnippetModal/index.jsx

...ages/GeneralSettings/EmbedConfigs/NewEmbedModal/index.jsx

frontend/src/pages/GeneralSettings/PrivacyAndData/index.jsx

frontend/src/pages/GeneralSettings/Security/index.jsx

frontend/src/utils/chat/markdown.js

frontend/src/utils/chat/themes/github-dark.css

frontend/src/utils/chat/themes/github.css