HTML, 198200 Bytes
<mark style="background-color:blue"id="a"></mark><mark id="b"></mark><script>document.getElementById("a").innerHTML=document.getElementById("b").innerHTML=(" ".repeat(78)+"\n"+"<br>").repeat(26)</script>
Those poor Ukrainians... 😔
You should click on the link to render the full page after running the code.
Prints the Ukrainian flag (although it's missing a trident unfortunately) using <mark> tags to render the colours. Uses a script to insert the text. Uses U+2001 EM QUAD as a blank character.