Bootstrap .btn is usually used with a .btn-something class to set the color of the background, text, hover effects etc.
Several .btn usages omit this, then apply direct styles to achieve the same effect.
This causes issues when the colors are changed.
e.g. if you set the preset to "plain" in the SCSS theme settings, then the light-gray secondary that Moodle sets is replaced with the standard Bootstrap secondary color.
The new footer button with the ? in a circle becomes hard to read, because the color of the text was hardcoded. If btn-secondary had been used, the text/icon would have automatically adjusted to become white.
The new toggler buttons similarly hard code a light gray background, while inherting the black text color from the default button.