Embedded mode improvement

[TBG-FR]

This PR fixes #94 and #158 :

• Selected theme (dark, light, auto) is now always applied in embedded mode
• If top navbar is enabled in embedded mode, the dark mode toggle button will be added to this navbar

Details:

• The script will now search for a sidebar, or a top navbar if the sidebar is not found
• When the script couldn't find a sidebar/navbar, there was an infinite loop, and we would never reach the "apply theme" part of the code, so I moved it outside the function (and I also added a retry count to avoid this loop).

[TBG-FR]

@pbaity Just noticed I created this branch from my 4.3.0-develop instead of your repo master, so I think it'll better to merge #162 first 😅

[pbaity]

@TBG-FR No problem, I do that by accident all the time! Would you mind updating this with master's changes so it's clear which changes are part of this PR? (It may just be the JS changes, but if you made any CSS changes, that way it will be clear which were part of this PR.)

[TBG-FR]

Done 😉 Thank you for your quick answers !

[pbaity]

@TBG-FR This works great! Thanks for the incredible feature!