Sorry, my skills are limited in this area. I want to change it only in the index page, where the posts are, nothing else.
I don't know where is the code, do you mean this?
Theme Name: Kirumo
Theme URI: http://kirumo.johnregan3.com
Author: John Regan
Author URI: http://johnregan3.me
Description: Kirumo is a simple reponsive blogging theme with an elegant sliding mobile menu. This theme supports custom background colors and images, as well as a custom accent (link) color. One if its unique features is a full-width page template with a horizontal row of three widgets below. It also features formatting for all post formats, RTL layouts, and vertical featured images. Download Kirumo and take it for a test drive!
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Text Domain: kirumo
Domain Path: /languages/
Tags: black, custom-colors, custom-header, custom-menu, dark, featured-images, fluid-layout, full-width-template, gray, left-sidebar, post-formats, responsive-layout, right-sidebar, rtl-language-support, sticky-post, threaded-comments, translation-ready, two-columns, white
Kirumo WordPress theme, Copyright (C) 2014 John Regan
Kirumo WordPress theme is licensed under the GPL.
Genericons font is licensed under the GPL and is available from http://genericons.com.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License, version 2, as
published by the Free Software Foundation.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
Kirumo is based on Underscores http://underscores.me/, (C) 2012-2013 Automattic, Inc.
Resetting and rebuilding styles have been helped along thanks to the fine work of
Eric Meyer http://meyerweb.com/eric/tools/css/reset/index.html
along with Nicolas Gallagher and Jonathan Neal http://necolas.github.com/normalize.css/
and Blueprint http://www.blueprintcss.org/
@import url( "inc/css/base.css" );
@import url( "inc/css/mobile-menu.css" );
@import url( "inc/css/kirumo.css" );
@import url( "inc/css/structure.css" );
@import url( "inc/css/genericons.css" );