If using Universal Rendering and/or GBC, the answer should be yes via Customisation.
The key point is finding the appropriate selector and then adding some customisation.
Placeholder is a little different,
https://www.w3schools.com/howto/howto_css_placeholder.asp but it should be a case of something like the following if the browser supports it
::placeholder {
color: red;
}
In case of HLine component, the trick again is finding the appropriate selector, Developer Tools should shows that as .gbc_HLineWidget and that the line is being drawn by use of border top. So
.gbc_HLineWidget {
border-top-color: red;
}