Change header style to dynamic on only 1 page without

I've a question regarding this page:

https://www.cableparkaquabest.nl/board-balance-day

I wanted to change the header style for only 1 page and found this code on this forum:

<style>
  #mainContent { display: none; } 
  
.header-actions .btn {
color: #fff !important;

}
  
<style>

I've added this code to the page header and it works. the only thing is that all my H1 headers are changed to black, also on other color themes in the page. For example: In the footer you see 'nieuwsbrief' which turned out black on this page. But the H1 in the header needs to stay black. Is there somebody who can help m out with this one

You missing a closing </style> tag

<style>
  #mainContent { display: none; } 
  
.header-actions .btn {
color: #fff !important;

}
  
</style>

Also, I don't think that code will run, because it has .btn class is correct only.