Do I need any programming knowledge(PHP) to customise the THESIS framework well or can I get away with XHTML/CSS? Thanks
A basic knowledge of Php will always be needed for any theme framework, including Thesis. Most frameworks offer a way to tap into their functionality via action and filter hooks. Learning how to use the hooks will require some sort of Php skills to bend them to do what you want. As an example, common framework hooks would be the byline (author, date, no of comments) above the post and the meta information (category, tags) at the end of post. Internally, frameworks have a function that grabs the information from the database, formats it into a string of Html, then prints it to the screen. You can use a filter hook to modify the generated snippet of Html, lets say, to remove the tag list from the meta box at the end of the post. One thing about Thesis, which I could never understand, is the theme administration panel for altering the color and font scheme. This control panel lets you input color numbers and font sizes, then transforms that information into a dynamic CSS stylesheet. This may be fine for the uninitiated who know nothing about stylesheets, but if you prefer to use a CSS editor and alter the styling yourself, then this is just a piece of useless skin (pardon the pun). Thesis markets itself as SEO-optimized out of the box. While this is true, most frameworks worth their salt are SEO optimized and/or offer backend options to alter some of that functionality (such as defining the format of the HEAD TITLE element). IMO, any framework offering this as a selling point is just a gimmick. You can make a "childtheme" from the stock TwentyTen theme, add in Yoast's SEO plugin and have an SEO optimized theme to start working from. Personally, I use the Hybrid Theme framework. SEO-wise, it is as good as any other framework, maybe more so in some areas. It has undeniably the best breadcrumb coded function that you can find, post edit boxes for your Title, Meta Description, Meta Keywords (or allows you to turn it off if you are using other SEO plugins), pre-packaged stylesheets and easy templating options for 2-column, 3-column, float em left or right, no columns, enhanced widgets, hooks/filters for every section on the page, and more.
If you want to modify thesis framework then it's better to have good knowledge on PHP. However, you might be able to play around with design aesthetic with knowledge of XHTML/CSS and a bit understanding of PHP.