You could copy the theme, modify the svg and switch to the new theme in System: Settings: General# cp -r /usr/local/opnsense/www/themes/{opnsense,custom}Edit.../usr/local/opnsense/www/themes/custom/build/images/default-logo.svgAnd set "custom" theme from the GUI.
<?php if (file_exists("/usr/local/opnsense/www/themes/{$themename}/build/images/default-logo.svg")) : ?> <img src="<?= cache_safe("/ui/themes/{$themename}/build/images/default-logo.svg") ?>" height="30" alt="logo" /> <?php else : ?> <img src="<?= cache_safe("/ui/themes/{$themename}/build/images/default-logo.png") ?>" height="30" alt="logo" /> <?php endif ?>
<?php if (file_exists("/usr/local/opnsense/www/themes/{$themename}/build/images/default-logo.svg")) : ?>
<?php else : ?>
<?php endif ?>
<svg xmlns="http://www.w3.org/2000/svg" width="150" height="30" viewBox="0 0 150 30"> <text x="10" y="25" font-family="Arial" style="fill:orange" font-size="24"> HOSTNAME </text></svg>