Tiki uses basically two style sheets to produce the layout and look of the site. One is tiki_base.css, which covers things that are unique to Tiki or otherwise outside the scope of bootstrap.css. The other, because Tiki now incorporates the Bootstrap CSS framework, is essentially a bootstrap.css variant.
An easy way to make a Tiki theme is to simply find or create a style sheet that is equivalent to bootstrap.css. That is, the file needs to contain all the rules in a default bootstrap.css, but with different values (colors, sizes, etc.) because in Tiki it will be laying out and styling a Bootstrap 3 web page.
getbootstrap.com/customize - The original Bootstrap theme customizer - input values for everything.
Bootstrap Magic - Input values for variables and see effect in real time; can export Less variables or CSS.
Bootstrap Live Customizer - More complete options, including panel colors
PaintStrap - Set of 7 color variables only
BootSwatchr - Edit variables.less and see effect in real time in sample page, but you have to know (or spend time discovering) where the code lines and page objects are. Can export Less and CSS.
StyleBootstrap - Site was offline on a January 23, 2016 check.
Visigene - Simple visual editor but pretty thorough effect (panels, etc. are styled).
...and more. If these don't seem to work correctly in one browser, try another.
Keep in mind that the style sheet that's produced this way will cover the color palette, font size and family, some padding and margins, button styles, and so on, but won't reach all of the page elements or every feature detail. Since this style sheet is made externally with no awareness of Tiki's specific HTML, it won't provide complete styling of the site. Some custom CSS rules may need to be added in the Look and Feel admin page's "customization" area. Or for more extensive styling, one of the other theme-making methods should be used.
Bootstrap style sheets made for other web applications such as WordPress may be able to be used with Tiki but the HTML they were written for is quite different from Tiki's, so considerable modification will probably be necessary. We plan to test and document this in the near future.
The found or created file can be placed anywhere, even in the Tiki site's file gallery. Then select "Custom theme by specifying URL" in the "site theme" field on the Look and Feel admin page (tiki-admin.php?page=look). And finally, input the URL in the "Custom theme URL" field. After a refresh (and possibly clearing the site's caches) the site should show the effects of the new style sheet.