Links in Table Headers do not show clearly when in modern theme

[YongBoLiu]

Describe the bug

A clear and concise description of what the bug is.
In a cacti plugin, I want a link in the cacti table header. After add a link tag in the html_start_box() call, it doesn't show clearly when in modern theme, while it's ok in other themes.

To Reproduce

Steps to reproduce the behavior:

1. call html_start_box() with a link as below,

html_start_box("User Filter for User [<a href='busers.php'> Batch Users</a>]" , '100%', '', '3', 'center', '');

2. Check the UI under the modern theme. It's not clearly, because the link text color is almost the same as background.

3. Check the UI under the other themes, it shows well.

Expected behavior

A clear and concise description of what you expected to happen.

Screenshots

If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information)

• OS: [e.g. iOS]
  Linux

• Browser [e.g. chrome, safari]
  Firefox and Chrome.

• Version [e.g. 22]

Smartphone (please complete the following information)

• Device: [e.g. iPhone6]

• OS: [e.g. iOS8.1]

• Browser [e.g. stock browser, safari]

• Version [e.g. 22]

Additional context

Add any other context about the problem here.
Not clearly as below,

[YongBoLiu]

It's ok under other themes, as below,

[YongBoLiu]

Related PR#3865, #3866