You can try this work around. Export the theme for the Tools menu. Rename the file to .zip,¨ In the zip-packager you will find theme configurationfiles as .xml files. You will see elements like this <style name="borderColor" value="11447982" /> Edit the files carefully and rename and import the zip-package again. I don't know if you can change the color of the specific border, but it is worth a try. I have used this to change the color of the OLAP-tables.
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