If you want to alter all <<button>> macros everywhere, but only them, use:
.macro-button {
/* style properties */
}
If you want to alter all <button> elements within the main passage display area, created by <<button>> or not, use:
#passages button {
/* style properties */
}
If you want to alter all <button> elements everywhere, use:
button {
/* style properties */
}
NOTE: SugarCube's UI bar menu "buttons" are not actually <button> elements and will not be affected by any of the above.