css - How to use less with php

i'm using wordpress and i would like to be able to change all the same color of my website by changing one php function on my admin panel. I tried to do that on my custom-style.php but it don't work :

@main_color: <?php the_field('main_color', 'option'); ?>;
a{
color: @main_color;
}
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I also tried this way on my header.php :

<style type="text/css">
@main_color: <?php the_field('main_color', 'option'); ?>;
</style>
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