A whole Bunch of 18 FREE and responsive OpenCart v.1.5.x Themes have been released just recently by panforest.net.
All free responsive Themes by Panforest.net:
https://www.opencart.com/index.php?rout ... forest.net
All the Themes are very simple to install and use, None of them ADD's anything anywhere else.
Just upload the Theme Sub into the Theme Folder, and after 'setting' a chosen Theme as DEFAULT
in the Admin Section, it just works!

According to the Contributor, the Themes have all functionalities of a normal opencart default Theme.
All Extensions by Panforest.net:
[urlhttps://www.opencart.com/index.php?route=market ... forest.net[/url]
Features :
• Responsive themes
• Compatible with the most common browsers: FireFox 1.5-3.5.6+, IE9+, Safari 3.2 +, Opera, Google Chrome
• Compatible with Open Cart Version v1.5.5.x, v1.5.6.x
• Designed with luxurious colors.
• Suitable for any kind of eCommerce shop
• Supports multiple languages, multiple currencies
• SEO Friendly
• Well documented for installation
Be aware:
• Product Image Settings are not ALL created responsive by DEFAULT! (But this can be added easy!)
• Some CSS Style Adjustments may be required, to place Stuff, to stay away a little from page sides, visually !
Themes are free for download as they come, and not supported!
(But this should not be a big Problem, after all...

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First Test Results:
They look good, and they should serve well, after playing around a little in the CSS Stylesheets, to make them look a little more 'clean' on the left/right Sides!

All Themes can be seen here, just use the Theme Switcher, to switch between the different Themes:
http://www.bigmax.ch/
I have just installed all of them, and made no Tests, really. But it's great News, to have so much Choices,
all of a sudden, to mock up existing OC v.1.5.x based Versions to yet be fully responsive! And so far, they all installed and work fully error free!
Good Luck

Ernie
PS: Be aware, that IMAGES are NOT responsive in all those free Themes, therefore, do not define images over 500px max.width size, of they will not be visible in full, if minimum Browser/Smartphone Resolution View is planned to be below this Value.
Be aware, that, if you already have another 'Contributor' Custom Theme installed, you may experience slight Problems, i.E., with a distorted Category Pagination Section, due to possibly some System/Controller/e.t.c., - Settings, created / modified by certain other responsive Theme-Extensions, offered by some Theme-Contributors. Such Custom Themes usually also contain Subs, VqMod's, and/or Files, placed outside of the Theme Sub, just to mention this.
In such a Case, you should de-activate all MODULE / VQMOD Functions, related with the possibly problemsome other Custom Theme, installed, before trying out one of the Themes, mentioned here.
But since those mentioned here all are created 'equal', technically, and don't use any VqMod's, System- or Controller-Files, other than the DEFAULT One's, they will NOT harm each other in any way, in case, you upload multiple of them at once !
Just to mention this, I had those Problems, because I had two other, different THEME-BRANDS installed already, when I started my Tests. But the OC DEFAULT THEME is no Problem anyway, by Nature of Things!

See the Image, with the Category Section, displaying such a Problem.