Ok if you are using a white background (like a screenshot i saw in your other post) then understandably there will be an issue with reading the clock.
The issue I mentioned in my previous post was thus unrelated.
You have a couple of ways to improve this:
1. Create custom templates, find the variables Fanvil uses to control font color, edit them and redeploy all the phones using the custom templates. On top of this extra work, you will have to be aware that 3CX does not provide support when using custom templates, and you also need to update the template manually whenever we release an update to our default one. If you have issues with your phones and wish to raise a case with support, you will have to revert them back to using our default template, and replicate the issue using this method first. This is the least desirable solution I would imagine and with the most administrative overhead.
2. Alternatively, just edit your png file to have a darker rectangle around the area where the clock is, hence making the default white clock font easily legible. No modifications necessary, very easy to do.
Hint: you can log into a phone using its web UI, download a screen capture while your existing background is installed, and this will make it easier to know where to place your rectangle for when you edit your logo file.