Tiki uses basically two stylesheets 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, and that are global and don't need theme color variables, etc. applied. The other stylesheet, because Tiki incorporates the Bootstrap CSS framework, is essentially a standard bootstrap.css variant but with additional CSS rules for use with Tiki.
An easy way to make a Tiki theme is to simply find or create a stylesheet that is equivalent to bootstrap.css (making sure that the file works correctly with the Bootstrap version used at the Tiki site).
About the results
Keep in mind that such a "generic" stylesheet 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.
The found or created generic bootstrap.css 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.
Some custom CSS rules may need to be added in the Look and Feel admin page's "customization" area. This method isn't recommended except as a quick test to see how the stylesheet works with Tiki. 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. A better method would be to start from scratch with a new theme, copying the generic variables into the new Tiki theme's variables.scss file as appropriate, as described on the page about making a new Tiki theme.