My brain refused to work at first, but then I solved the problem anyway, and I felt ashamed of what I had asked. In any case, someone might need an answer.
First of all you need to add blocks for that lights:
<button class="header__donate-button">donate
<div class="yellow-light"></div>
<div class="blue-light"></div>
</button>
After that, simply adding css styles, where lights are positioned absolute, and overflow in parents block is hidden
header__donate-button {
width: 11.875rem;
height: 3rem
color: #ffffff;
background: #1A1A1A;
border-radius: 15px;
position: relative;
overflow: hidden;
.yellow-light {
position: absolute;
width: 84px;
height: 84px;
bottom: -3rem;
left: -3rem;
background: #e6bc50;
filter: blur(50px);
}
.blue-light {
position: absolute;
width: 84px;
height: 84px;
top: -3rem;
right: -3rem;
background: #50b0e6;
filter: blur(50px);
}
}