Poor contrast for progressbar text

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same problem, before opening a new one.
• This issue only relates to a single bug. I will open new issues for any other problems.

Describe the bug

As initially reported in Jacalz/rymdport#115, the widget.ProgressBar component has poor contrast for the text in the case where the progress bar hasn't covered the progress background yet. Both the light and dark themes exhibit poor contrast initially but good contrast later on. See screenshots below.

How to reproduce

1. Open up fyne_demo.
2. Go to the Widgets -> Progress page.
3. Notice that the text contrast is poor initially but turns good once the progress reaches ahead of the text.

Screenshots

Dark theme:

Poor contrast for text in the dark region of the progress bar:

Good contrast for text in the light region of the progress bar:

Light theme

Poor contrast for text in the light region of the progress bar:

Good contrast for text in the dark region of the progress bar:

Example code

See fyne_demo.

Fyne version

v2.4.5

Go compiler version

1.22.3

Operating system and version

Fedora Silverblue 40

Additional Information

No response