Looking at the CSS (the webkit prefix appears striken out with Firefox but can be seen nonetheless), Bootstrap is the last to add
-webkit-appearance: button. This would have to be overridden by a
none as far as I can see. I think you should maybe place your custom stylesheet after it in the document markup.
You might run into more trouble otherwise as well, Bootstrap has a bunch of defaults that could cancel out what you’ve coded yourself.
Opera’s “just another browser” with a minor market share, very similar to Chrome (also webkit engine). I always include it in my check anyway – it rarely has any issues (unlike early IE or Android, the two trickiest browsers when it comes to compatibility).