Omit @private and type information from JsDoc since that's captured in the TypeScript code.

Rather than having one function perform two behaviors with an option, I generally prefer breaking it up into separate functions. Also, most of these can be module-level functions since they are pure functions, e.g.

class StyleManager {
  setStyle(key: string, href: string) {
    getLinkElementForKey(key).setAttribute('href', href);
  }

  removeStyle(key: string) {
    const existingLinkElement = getExistingLinkElementByKey(key);
    if (existingLinkElement) {
      document.head.removeChild(existingLinkElement);
    }
  }
}

function getLinkElementForKey(key: string) {
  return getExistingLinkElementByKey(key) || createLinkElementWithKey(key);
}

function getExistingLinkElementByKey(key: string) {
  return document.head.querySelector(`link[rel="stylesheet"].${getClassNameForKey(key)}`);
}

function createLinkElementWithKey(key: string) {
  const linkEl = document.createElement('link');
  linkEl.setAttribute('rel', 'stylesheet');
  linkEl.classList.add(getClassNameForKey(key));
  document.head.appendChild(linkEl);

  return linkEl;
}

function getClassNameForKey(key: string) {
  return `style-manager-${key}`;
}

Prefix css classes with docs-

We should make the theme chooser aria-hidden="true" and ensure that the button is not in the tab order since a screen-reader user won't get any benefit from changing the app colors.

We should omit assets/ on each entry and add it in the function call.

Can these be referenced directly from node_modules rather than copying them to assets?

How do you feel about the accent stripe? I'm not particularly fond of it.

CUSTOM_ELEMENTS_SCHEMA ideally shouldn't be here; why was this needed?

InlineSvgModule (camelCase even for acronyms)

Commented line

Commented code

I'd like to avoid /deep/ completely, instead preferring to add a global style (or, barring that, turning off view encapsulation for the component)

We prefer not to prefix interfaces with I (here and elsewhere)

What's this require doing here?

Prefer arrow functions to using bind

Commented code

Add a TODO here like

<!-- TODO: replace use of `md-menu` here with a custom overlay -->