The configuration options you posted do not include any Back Button options but if I add the following to a Juicebox-Pro v1.4.0 test gallery using your configuration options, the Back Button appears black on a white background.
backButtonPosition="TOP"
backButtonUseIcon="TRUE"It sounds like you might be using an older version of Juicebox-Pro (although if you just downloaded it yesterday, this is unlikely) or there may be a conflict with some custom CSS code on your web page (if you have embedded your gallery into an existing web page alongside other content).
Please check for these two possibilities and if you continue to experience difficulties, please post the URL to your gallery so that I can take a look and help further.
(You can check which version of Juicebox-Pro your gallery is using by looking at the comments at the top of the 'jbcore/juicebox.js' file and you should be able to see if there are any custom CSS rules being applied to the Back Button using your browser's developer tools.)