How to store css into mysql database and call it back to be displayed on html

I am not sure if storing CSS in database is a good idea, but since you are explicitly requesting for it then you might follow this approach.

TABLE: styles

| id | selector | style_name   |
|----|----------|--------------|
| 1  | body     | align-center |
| 2  | .d-flex  | display-flex |
| 3  | #badge   | custom-badge |

TABLE: styles_css

| id | style_name   | property     | value     |
|----|--------------|--------------|-----------|
| 1  | align-center | text-align   | center    |
| 2  | display-flex | display      | flex      |
| 3  | custom-badge | color        | red       |
| 4  | custom-badge | font-size    | 1.5rem    |

Then when you need the styles for your HTML, you can join these tables to get CSS styles.

SELECT `property`, `value`
FROM `styles_css` 
LEFT JOIN `styles`
ON `styles`.`style_name` = `styles_css`.`style_name`
WHERE
`styles`.`selector` = '#badge';

This will give you

| property   | value    |
|------------|----------|
| color      | red      |
| font-size  | 1.5rem   |

Example:

<?php
    // prepare statement and bind params(PDO would be a good option)
    $sql = "
        SELECT `property`, `value`
        FROM `styles_css` 
        LEFT JOIN `styles`
        ON `styles`.`style_name` = `styles_css`.`style_name`
        WHERE
        `styles`.`selector` = '#badge';
    ";
     //after execution, something like $styles = $db->exec($sql);
    foreach($styles as $property => $value) {
        $css = "{$property} : {$value}; "; 
    }
    // remove trailing semi-colon
    $css = rtrim($css, ';');
?>