Change Font Color on MailChimp Subscribe Button

This simple detail is baffling me and I hope you can help.

I want to change the font color on the 'subscribe me to this blog' button at https://resumerelief.com/blog/ from black to #585858.

My MailChimp subscribe.html page code is pasted below.

I'm questioning whether I can add a bit of code on that page to accomplish the color change or if I need to do it in CSS - and if it needs to be done in CSS, what CSS file needs to be changed?

Thanks!

<perch:form id="subscribe" app="perch_mailchimp" double-optin="true" send-welcome="true">

    <perch:success>
        <p>Thanks so much & head to your email box to confirm subscription!</p>
    </perch:success>

    <div>
        <perch:label for="email">email</perch:label>
        <perch:input id="email" required="true" size="17" type="email" mailer="email" />
        <perch:error type="required" for="email">Required</perch:error>
    </div>
    <div>
        <perch:label for="firstname">1st name</perch:label>
        <perch:input id="firstname" required="true" size="17" type="text" mailer="FNAME" />
        <perch:error type="required" for="firstname">Required</perch:error>
    </div>
    <div>
        <perch:label for="lastname">last name</perch:label>
        <perch:input id="lastname" required="true" size="17" type="text" mailer="LNAME" />
        <perch:error type="required" for="lastname">Required</perch:error>
    </div>
    <div>
        <perch:input type="submit" value="subscribe me to this blog" color="#585858" id="btnsubmit" />
        <perch:input type="hidden" value="1" id="confirm" mailer="confirm_subscribe" />
    </div>  

</perch:form>