You'll have a bit more control if you set the Use Theme property to no, but AFAIK, the image package is what you get and it always seems to change from version to version. If you do use custom images as suggested, they will need to have transparent backgrounds or at least the background will have to match. If users can change the Access theme it will probably clash with whatever you design, then there's the problem of making sure images are available to all users, or you have to embed them in the controls. Embedding can add significantly to your file size. You might want to consider taking off the theme constraint and colouring everything else to suit the arrow colour.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.