fix: improve contrast ratio for button placeholder

[lex111]

Summary

Currently there is a11y error in light mode related to insufficient color contrast in the placeholder button.

Result

Checker: https://webaim.org/resources/contrastchecker/

Before	After

cc @francoischalifour

[lex111]

Just friendly reminder, since I would like to see this fix in new version.

[Shipow]

Hi @lex111 , we've acknowledged the problem but the fix seams to break some important hierarchy info so we need to rework on this. Thanks for pointing this. When do you plan to release a new version?

[lex111]

I don't know exactly, but I wanted to do it as soon as possible. So I will wait for your action.