You can get themes for your stock keyboard through the LG Smartworld app but now thanks to Ashrockr over at XDA there are some really nice colored and Android L style themes available. Here's a link to a "How To" article with a link to Ashrockr's thread over at XDA.
Get Exclusive Keyboard Themes on Your LG G3—No Root Required ? LG G3
Get Exclusive Keyboard Themes on Your LG G3—No Root Required ? LG G3