boardrules.tpl line 20. The 'form-button' class is on the check box, not the button:

Quote
<input type="checkbox" name="agree" value="1" id="agree" class="form-button" /> <label for="agree">{$lang.RULE_ACCEPT_COMMIT}</label>
<br />
<input type="submit" value="{$lang.PROCEED}" />
Should be:

Quote
<input type="checkbox" name="agree" value="1" id="agree" /> <label for="agree">{$lang.RULE_ACCEPT_COMMIT}</label>
<br />
<input type="submit" value="{$lang.PROCEED}" class="form-button" />