Tap the Guest icon at the top right ... Yes, updating Chrome or switching to the default theme may restore the button, as it is affected by custom configurations or experimental features.
Recent updates introduced picture-in-picture support and an "Open in browser" shortcut for Chrome Custom Tabs (CCTs ... sit beside the 'Open in browser' icon. First introduced all the way ...
Practically speaking, there is no reason to use a theme to your Google account. They don't change the way you use the browser or affect your interaction with Gmail, Google Docs, or any of the ...