Yes. 
By chance, I was just reading the KDP official formatting guide this morning (you know, for a little light reading!) and they actually say that you should not specify colour, size, or font, alignment or anything else for body copy. Indent and paragraph spacing is about all you should specify. You should leave the rest out so that the device can set the body copy at whatever the device's "default" setting is.
So not only is it a way to shave off KBs, but it results in better formatting for your book.
Be aware that if you use conversion software like Calibre, it may add some of that stuff back in, even if it is not present in the original HTML. Not sure if it does in the latest version, but it might. What I do is convert to EPUB and then check in Sigil (free epub editor) to make sure all that stuff is fine. Then convert the epub to mobi using KindleGen (or you can just upload it direct to KDP)
By chance, I was just reading the KDP official formatting guide this morning (you know, for a little light reading!) and they actually say that you should not specify colour, size, or font, alignment or anything else for body copy. Indent and paragraph spacing is about all you should specify. You should leave the rest out so that the device can set the body copy at whatever the device's "default" setting is.
So not only is it a way to shave off KBs, but it results in better formatting for your book.
Be aware that if you use conversion software like Calibre, it may add some of that stuff back in, even if it is not present in the original HTML. Not sure if it does in the latest version, but it might. What I do is convert to EPUB and then check in Sigil (free epub editor) to make sure all that stuff is fine. Then convert the epub to mobi using KindleGen (or you can just upload it direct to KDP)