Hi Vicky,
This template is built with T3 framework which implements Bootstrap, you can change it globally via the LESS file:
root/templates/ja_cityguide/less/variables.less
Look for the declaration for Typography of template:
@font-size-base: 14px;
@font-size-large: 14px; // ~18px
@font-size-small: ceil(@font-size-base * 0.85); // ~12px
@font-size-h1: 60px;
@font-size-h2: 36px;
@font-size-h3: 20px;
@font-size-h4: 16px;
@font-size-h5: 14px;
@font-size-h6: 12px; // ~12px
@line-height-base: 1.714286; // 24 /14
@line-height-computed: floor(@font-size-base * @line-height-base); // ~24px
@headings-font-family: @font-family-base;
@headings-font-weight: 600;
@headings-line-height: 1.3333;
@headings-color: #343648;
and change the font size bas variable as you wish.
Once done, you go to Backend > Extensions > Templates > JA CityGuide template style > General tab > and see the status of 'Development Mode' option. If it's ON, then after changing LESS file, just refresh front-end of your site to see the change.
In case, this dev mode is OFF, you will need to compile LESS to CSS (Remember to backup all CSS files first as when compiling, they will be overwritten).
Regards