HCM: Fix accessibility of post-icon in form select in high contrast mode (dark theme)

[alionazherdetska]

Description:
The post-icon used in the form select is not visible in high contrast mode with the dark theme because it is rendered in black, blending into the background. This issue does not occur in the light theme, where the icon is displayed correctly.

Steps to Reproduce:

1. Enable high contrast mode in the browser or system settings.
2. Switch to dark theme.
   https://support.microsoft.com/en-us/windows/turn-high-contrast-mode-on-or-off-in-windows-909e9d89-a0f9-a3a9-b993-7a6dcee85025#:~:text=Select%20the%20Start%20button%2C%20then,like%20links%20and%20button%20text.
3. Navigate to the Form Select page.
4. Observe that the post-icon is not visible.