Please follow the below steps to add javascript and CSS file in the shopware 5 themes.
For CSS
File Structure
themes>Frontend>Custom>frontend>_public>src>css.
Step 1: Create a new file called style.css in css folder.

Step 2: Click on the style.css file and add some css.

Step 3: Now go to the Theme.php file .To add the file paths to the $css
variable.

Step 4: After saving the file. Go to the Store-Front to make sure it changes the background color.

For JavaScript
File Structure
themes>Frontend>Custom>frontend>_public>src>style.js
Step 5: Create a new file called style.js in js folder.

Step 5: Click on the style.js file and add some js.

Step 6: Now go to the Theme.php file .To add the file paths to the $js
variable.

Step 7: After saving the file. Go to the Store-Front to make sure it work.

Please contact us at manish@bay20.com or call us at +91-8800519180 for any support related to shopware 5. You can also visit the Shopware 5 development page to check the services we offer.