You would have noticed that If your parent site has a theme applied to it and if you create a new subsite, the subsite will not inherit the theme from the parent site. Well, here is a quick solution to this.
After you apply the theme to your top-level or parent site do the following :
1. View the HTML source of the SharePoint page using the theme.
2 Look for a link tag containing the .css file for the applied theme. It will look something like this:
<link rel="stylesheet" type="text/css" id="onetidThemeCSS" href="/SITE/_themes/reflector/Ref1011-65001.css?rev=12%2E0%2E0%2E4518″/>
3. Copy the href value to your clipboard or to a text file if you like. It should look something like:
/SITE/_themes/reflector/Ref1011-65001.css
4. On your top level site, go to the Site Actions > Site Settings > Master Page screen.
5. Scroll to the bottom where the Alternate CSS URL section is. Select the "Specify a CSS file…" option.
6. Paste the path copied from the href above into the text box.
7. Check the Reset all sub-sites to inherit this alternate CSS URL option.
9. Click OK.
And you are Done!