Better support of dark theme in WebViews #178

The WebView applies different styling to the web page under light, dark, high-contrast mode (see https://code.visualstudio.com/api/extension-guides/webview#theming-webview-content). Currently, we ignore such default styling.

However, under some circumstances (e.g #172) the default styling of certain VSCode theme under dark mode seems to make the web pages produced in R hard to read.

We may need to find a way to either override all default styling of the WebView, or respect the theme mode and adjust the HTML content accordingly.