By default all the blocks are assumed to be cacheable, but you can prevent chaching of any specific block by setting cacheable attribute as false in the XML file.
In Magento_Customer::view/frontend/layout/customer_account_index.xml file, you can find following piece of code
<block class="Magento\Customer\Block\Account\Dashboard\Info" name="customer_account_dashboard_info" as="info" template="account/dashboard/info.phtml" cacheable="false"/>
<block class="Magento\Customer\Block\Account\Dashboard\Address" name="customer_account_dashboard_address" as="address" template="account/dashboard/address.phtml" cacheable="false"/>
You can see that the above block are set as non-cacheable by using cacheable="false" attribute.
Similarly you can also do the same with your code and then in your phtml file (customCss.phtml file), you can load the CSS file normally like following -
<link rel="stylesheet" type="text/css" media="all" href="<?php echo $_helper->yourCssFile()?>">
And you can get the dynamic url of your CSS file using yourCssFile() function. As the page will not be chached so every time, new CSS file will get loaded. But there is a price of doing this. cacheable="false" will disable the cache for whole block and will make the page slower to load.
For more information you can visit the following URL -: